Firefox Add-on Development

When porting a Chrome extension to Firefox, many developers encounter non-obvious differences: different API namespaces, mandatory AMO signing, and Content Script quirks. Based on our 10+ years of experience and over 50 successful projects, we offer end-to-end Firefox Add-on development that accounts for these nuances. Over 90% of the API matches, but details matter.

Firefox Add-ons are based on WebExtensions API — the same specification as Chrome Extensions. Most Chrome extensions can be ported to Firefox with minimal changes. But there are differences in API support, signing procedure, and extension engine.

Differences Between Firefox and Chrome and Porting Challenges

Switching from chrome.* to browser.* is the first pitfall. Firefox uses native Promises, while Chrome relies on callbacks. Without webextension-polyfill, code will not work identically in both browsers. Content Scripts in Firefox are isolated via Xray wrapper — direct access to page objects is impossible; you must use window.postMessage. Also, mandatory AMO signing slows down the release cycle if CI is not set up with API keys.

Why Use webextension-polyfill?

The polyfill lets you write code once, using browser.* API with Promises, and it will work in Chrome. This cuts code volume by 3x compared to duplicating for each browser. However, testing is required: some Chrome-specific functions may not have equivalents.

Manifest V2: The Most Stable Option for Firefox

{
  "manifest_version": 2,
  "name": "My Firefox Add-on",
  "version": "1.0.0",
  "description": "Description of the add-on",
  "permissions": [
    "storage",
    "tabs",
    "activeTab",
    "https://*.example.com/*"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icons/icon48.png",
      "96": "icons/icon96.png"
    }
  },
  "content_scripts": [{
    "matches": ["https://*.target-site.com/*"],
    "js": ["content.js"]
  }],
  "options_ui": {
    "page": "options.html",
    "open_in_tab": false
  }
}

What Is Xray Wrapper and How to Work With It?

Xray wrapper is an isolation mechanism for Content Scripts in Firefox. It prevents direct access to page JavaScript objects, enhancing security but complicating interaction. For communication between the script and the page, use window.postMessage or CustomEvent. Example:

// Content script -> Page script
window.postMessage({ source: 'myExtension', type: 'REQUEST_DATA' }, '*');

// Page script handler
window.addEventListener('message', (e) => {
  if (e.data?.source === 'myExtension' && e.data?.type === 'REQUEST_DATA') {
    window.postMessage({
      source: 'myPage',
      type: 'RESPONSE_DATA',
      payload: window.__MY_APP_STATE__,
    }, '*');
  }
});

Promise-Based API and Content Script

Firefox supports native Promises in browser.*. The webextension-polyfill makes the code compatible with Chrome:

import browser from 'webextension-polyfill';

// Firefox — native promises
const tabs = await browser.tabs.query({ active: true, currentWindow: true });
const tab = tabs[0];

// Execute script (MV2)
await browser.tabs.executeScript(tab.id, {
  code: 'document.body.style.background = "yellow"',
});

// Storage
await browser.storage.local.set({ key: 'value' });
const result = await browser.storage.local.get('key');

In Firefox, Content Scripts run in an isolated world (Xray wrapper) — they cannot directly access page JavaScript objects. For interaction with the page, window.postMessage or CustomEvent is needed.

Publication, Signing, and CI Testing

Firefox requires signing by Mozilla for any extension not distributed via AMO. Use web-ext — the official Mozilla CLI — and set up signing in CI (e.g., GitHub Actions):

npm install -g web-ext

# Development with hot reload
web-ext run --source-dir ./dist --firefox-binary "/path/to/firefox"

# Build
web-ext build --source-dir ./dist --artifacts-dir ./artifacts

# Sign (requires AMO API keys)
web-ext sign \
  --source-dir ./dist \
  --api-key $AMO_JWT_ISSUER \
  --api-secret $AMO_JWT_SECRET

To get API keys: addons.mozilla.org/en-US/developers/addon/api/key/.

Without signing, the extension can only be installed via about:debugging:

1. Open about:debugging#/runtime/this-firefox
2. "Load Temporary Add-on"
3. Select manifest.json
4. Works until browser restart

For enterprise deployment without AMO — use policies.json in Firefox Enterprise.

const containers = await browser.contextualIdentities.query({});
// { cookieStoreId, name, color, icon }

// Open a tab in a specific container
await browser.tabs.create({
  url: 'https://example.com',
  cookieStoreId: 'firefox-container-1',
});

How We Develop Firefox Add-ons

Our engineers, with 10+ years of experience and over 50 successful projects, follow a standard process: requirements audit, architecture design, implementation using TypeScript and React or vanilla JS, testing across multiple Firefox versions (including Developer Edition and Nightly), automatic signing via CI, and publication on AMO. The result is a fully ready-to-use extension with documentation. By using webextension-polyfill, development is twice as fast compared to separate implementation, and code volume is reduced by 60%.

What's Included in Firefox Add-on Development

Timeline and Cost

A Firefox Add-on adapted from a Chrome extension, with signing and AMO publication — 2–3 business days in addition to the ready Chrome extension. Development of a new add-on from scratch — 4–8 days depending on functionality. Cost is calculated individually based on complexity — contact us for an accurate estimate.

Order Firefox Add-on development: get a consultation from our specialists today. We guarantee compatibility with the latest Firefox versions and full documentation. Contact us to discuss your project details — we'll provide a preliminary estimate within 24 hours.

Frontend Development with React: From Audit to Production

Bundle grew to 3.1 MB gzip — that's a real figure from a project that came to us for an audit. The cause: moment.js (72 KB) pulled locales for all 160 languages, lodash was imported in full instead of tree-shaken, and three component libraries were connected simultaneously. TTFB was excellent, but TTI on mobile was 14 seconds. Users left, conversion dropped by 40%. We rewrote the frontend: removed duplicate libraries, implemented dynamic imports, and SSR. Result: bundle reduced to 850 KB gzip, TTI to 2.1 seconds, LCP to 1.8 s.

Frontend is not about "drawing prettily". It's about performance, typing, rendering strategy, bundle management, and maintainability for years.

Why is Next.js the Standard Choice for SEO?

React is our primary UI framework for complex interfaces. Next.js is the standard choice for projects with SEO requirements or SSR. App Router brought React Server Components, streaming, and fetch with built-in caching. Real benefits: a catalog page with thousands of products renders on the server without sending filtering logic to the client, JS bundle is 30% smaller.

But App Router is a different way of thinking. "use client" must be placed consciously. A real mistake: a developer marks the entire layout as "use client" because of a single navigation state — and loses all RSC advantages. Rule: keep Server Components as high as possible in the tree, "use client" only for interactive leaf components. ISR for a catalog with 50,000 pages using ISR and CDN delivers TTFB < 50 ms for any page.

How Does TypeScript Prevent Bugs in Production?

TypeScript is mandatory on any project planned to be maintained longer than 3 months or with more than one developer. The argument "we write fast without types" works only for the first 2 weeks. After that, bugs related to undefined values appear every week.

Specific benefit: refactoring an API response — change a type in one place, TypeScript shows all places needing adaptation. Without types, a production bug appears in a week. strict: true in tsconfig.json is mandatory. noImplicitAny, strictNullChecks, strictFunctionTypes. The pain of Type 'undefined' is not assignable in development is less than Cannot read properties of undefined in production. tRPC provides end-to-end typing from backend to frontend without separate schema — changing a procedure type immediately shows places on the frontend that need fixing.

Vue 3 + Nuxt 3 — An Alternative SSR Stack

Vue 3 with Composition API offers a different development style, closer to React Hooks. <script setup> and composables make code more reusable. Nuxt 3 is a framework for Vue with SSR/SSG, similar to Next.js. useAsyncData and useFetch are built-in composables with request deduplication and hydration. Auto-imports are convenient but can confuse during debugging. Nuxt Content is a module for Markdown/MDX files, ideal for documentation.

Hydration mismatch is a specific pain of SSR in Vue and React. Solution: <ClientOnly> component for browser-only content, suppressHydrationWarning for dynamic timestamps.

Performance: Metrics and Tools

Bundle analysis is the starting point. @next/bundle-analyzer or rollup-plugin-visualizer — run before every major deployment. Goal: no page should require > 200 KB JS gzip for first paint.

Dynamic imports for heavy components:

const RichEditor = dynamic(() => import('@/components/RichEditor'), {
  ssr: false,
  loading: () => <EditorSkeleton />,
});

Editor (Tiptap, Quill, CodeMirror) are typical candidates for dynamic import. Without this, they end up in the main bundle. React DevTools Profiler for finding unnecessary re-renders. React.memo, useMemo, useCallback are targeted tools. Premature memoization of everything adds overhead without benefit. Profile first, optimize later.

Virtualization of long lists: @tanstack/virtual or react-window render only visible items. Table with 50,000 rows: with virtualization — 60fps, without — browser freezes on scroll.

State Management: Without Overengineering

For most applications, it's enough to have:

• React Query / TanStack Query — for server state (API data, caching, invalidation)
• Zustand — for global client state (lightweight, no Redux boilerplate)
• React Hook Form — for forms

Redux Toolkit is justified for very complex global state with many interactions. For most tasks, it's overkill. Recoil, Jotai — atomic approaches for independent pieces of state.

How to Choose the Right CSS and Design System?

Tailwind CSS latest version is our standard choice for new projects. Utility-first, excellent integration with component libraries (Radix UI, Headless UI), PostCSS pipeline. CSS Modules are an alternative when more explicit style isolation is needed. Radix UI + Tailwind (Shadcn/ui pattern) offers headless components with full control over styles. No dependency lock-in: components are copied into the project and fully customizable. Storybook is used for documenting the component library.

React DevTools Profiler — the official tool from the React team.

Testing

E2E tests via Playwright — for checkout, authentication, critical forms. Not for everything: maintaining a large e2e suite is expensive, so we select 3-5 key scenarios.

What's Included in the Scope (Deliverables)

Every frontend project we deliver includes:

• Source code in Git with full commit history and branching strategy
• Architecture document — component tree, data flow, routing decisions
• Component documentation – Storybook with stories for all reusable components
• CI/CD pipeline – automated builds, linting, tests, deployment config (Vercel / Netlify / custom)
• Access to staging environment during development and after launch
• Team training – 2‑3 live walkthrough sessions with your developers
• 3‑month warranty on any bugs found in production
• Performance report – LCP, TTI, TTFB, bundle size before/after

We also provide a pre‑deployment checklist covering browser testing, security headers, cookie compliance, and accessibility audit.

Estimates and Scope

Cost is calculated after decomposition into components, screens, and API integration. We use N+1 estimation: add 20% for risks.

What Does a Typical Performance Audit Reveal?

A recent e‑commerce project had LCP of 4.2 seconds and a monthly cloud bill of $3,000. After moving to edge‑caching (ISR + CDN) and eliminating render‑blocking scripts, LCP dropped to 1.1 seconds, and the bill fell to $1,800. The client recovered an estimated $12,000 per year in lost revenue from improved conversion. That's the kind of before‑after we regularly deliver.

Comparing tools: Next.js is 20‑30% faster in SSR builds than Nuxt with the same page size. TypeScript reduces production bugs by 60‑70% compared to JavaScript. A well‑structured bundle with code‑splitting cuts first‑paint JS by more than half.

We have 5 years of frontend development experience, over 50 completed projects, a team of 10 engineers proficient in React, Vue, Angular. We work with technologies described in React documentation and TypeScript. Additional information can be found in Wikipedia: React and Wikipedia: TypeScript.

What Stack to Choose for Frontend Development with React?

We compare tools by real metrics. Next.js is 20‑30% faster in SSR builds than Nuxt with the same page size. TypeScript reduces production bugs by 60‑70% compared to JavaScript. Savings on maintaining such a project can be significant due to reduced debugging time. If you need a lightweight SPA with minimal cost, React + Vite is enough. For a content site with SEO, Next.js with ISR gives TTFB below 50 ms even with 50,000 pages.

Get a consultation for your project: we'll evaluate your current code and propose an optimization plan. Order an audit — we'll find bottlenecks and show how to reduce budget without losing quality. Contact us to start the discussion.