- [Firebug] (http://getfirebug.com/) - set of development tools which makes developer’s life easier
- [Measurelt] (https://addons.mozilla.org/en-us/firefox/addon/measureit/) - draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels
- [HttpRequester] (https://addons.mozilla.org/en-us/firefox/addon/httprequester/) - a tool for easily making HTTP requests (GET/PUT/POST/DELETE), viewing the responses, and keeping a history of transactions
- [Stylish] (https://addons.mozilla.org/en-US/firefox/addon/stylish/) - A tool to add custom styles for web pages. I personally use it to color the background of open tabs so I can distinguish which of the tab is development/test/accepptance/production version.
- [Ripple emulator] (https://chrome.google.com/webstore/detail/ripple-emulator-beta/geelfhphabnejjhdalkjhgipohgpdnoc) - multi-platform mobile environment emulator that is custom-tailored to mobile HTML5 application development and testing
- [IE TAB] (http://www.ietab.net/) - allows you to use Internet Explorer to display web pages in a tab. This is useful in many cases where a web page requires Internet Explorer features
- [ColorZilla] (http://www.colorzilla.com/) - advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies
- [Visual Event] (http://www.sprymedia.co.uk/article/Visual+Event+2) - an open source Javascript bookmarklet which provides debugging information about events that have been attached to DOM elements
- [Firebug Lite] (https://getfirebug.com/firebuglite) - Fork of Firebug aimed to provide better performance and cross browser functionaliy. Since it is javascript based some functionality which involves Browser is missing like Javascript Profiler/Debugger, Net panel, Context menus etc.