A browser extension that adds various web developer tools to a browser.
Best Tools For Firefox Extension
Browse 9 + Firefox Extension Tools

Web Developer

Omni
The all-in-one tool to supercharge your productivity ⌨️
Web Developer Checklist
Chrome and Firefox extension to discover problem areas in websites.
SelectorsHub
A FREE xPath plugin to auto generate, write and verify xpath & cssSelector.

TestCase Studio
It’s a browser extension that records the user actions performed on a web application in English Sentences.

QAide
The Test Step Recorder
WAVE
Web Accessibility evaluation tool

Form Filler
A customizable Chrome and Firefox extension that fills dummy text in all input fields in a page.

Bug Magnet
Exploratory testing assistant for Chrome and Firefox