Skip the "50 must-have extensions" lists. Here's a focused set of Firefox extensions that address real development workflows — tested and used regularly.
Debugging and Inspection
React Developer Tools
If you work with React, this is non-negotiable. Shows the component tree, props, state, hooks, and a profiler. Works the same as the Chrome version.
Available on AMO.
Vue.js devtools
Same for Vue — component inspector, state viewer, event tracking. AMO link.
Redux DevTools
If you use Redux for state management, the DevTools extension lets you inspect action history, time-travel debug, and inspect state. Works with most Redux-based libraries.
Axe DevTools
Automated accessibility testing. Scans the current page and reports WCAG violations with explanations and fix suggestions. Catches common issues: missing alt text, insufficient color contrast, keyboard focus problems.
Essential if accessibility matters to your project.
Network and API
RESTer
Lightweight REST client built into Firefox. Send HTTP requests, inspect responses, manage saved requests. Alternative to Postman for quick API testing during development — stays in the browser, no separate app needed.
GraphQL Network Inspector
If you work with GraphQL, this shows GraphQL queries and mutations in the Network tab with proper formatting and variable inspection.
Frontend Utilities
ColorZilla
Color picker with hex/RGB output. Eyedropper for any color on screen. Gradient generator. Useful for matching colors from designs or checking what a specific element's computed color is.
WhatFont
Click any text on a page to see its font family, size, weight, and line-height. Useful for matching designs or inspecting typography on competitor sites.
CSS Scan
(Paid, but worth mentioning) CSS Scan shows the complete computed CSS for any element on hover — no clicking, no DevTools panel needed. Dramatically faster for quick CSS inspection.
Privacy and Tracking
uBlock Origin
The standard. Also useful for development: temporarily disable to test your analytics, check what ads look like, or debug why something isn't showing up on your site.
Medium/Hard Mode in uBlock gives you dynamic filtering — block third-party scripts per-domain, which is useful for testing how your site behaves when scripts are blocked.
Privacy Badger
Learning-based tracker detection (from EFF). Useful to see what third-party tracking your site is sending — Privacy Badger will flag things uBlock Origin might allow.
Tab and Session Management
Tab Manager & Session Saver
For development workflows with many tabs: save named session snapshots. When you're debugging a production issue, you probably have 10+ tabs open. Save them as "prod-incident-oct16", close when done, restore if needed.
Multi-Account Containers
For testing multi-user flows: open separate containers for "logged in as user A" and "logged in as user B". Each container has an isolated cookie store.
Official Mozilla extension on AMO.
For Specific Tech Stacks
JSON Viewer (or JSON Formatter)
Makes raw JSON responses readable in the browser. When you visit an API endpoint, Firefox by default shows raw JSON text. A JSON viewer extension adds collapsible tree view, syntax highlighting, and search.
OctoLinker (for GitHub)
Turns module import statements in GitHub source code into clickable links that navigate to the imported module. Dramatically more useful for reading code on GitHub.
Refined GitHub
Dozens of UX improvements to GitHub: better PR diffs, keyboard shortcuts, improved code viewing. If you spend significant time on GitHub, worth installing.
What Not to Install
Multiple SEO toolbars. They run constantly, add toolbar clutter, and the data they show is available in the free tiers of purpose-built tools.
"All-in-one developer" bundles. These pack many tools together — most of which you won't use — and have a larger attack surface and higher overhead.
Closed-source, unreviewed extensions. Check AMO reviews and the extension's source policy before installing anything with access to your browsing data.
Effective extension setup for most web developers: uBlock Origin + React/Vue DevTools (if applicable) + RESTer + Axe + Multi-Account Containers + Tab Manager for session management. Everything else is situational.
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