1. Firebug: A Firefox plugin that allows inspect, edit and monitor CSS, HTML, JavaScript and Net requests in any web page.
2. FireCookie : An extension for Firebug that makes possible to view and manage cookies in your browser.
3. FireShot : An Add-on to Firefox that creates screenshots of web pages entirely. The captures can be quickly annotated.
4. Page Speed : An open-source project started at Google to help developers optimize their web pages by applying web performance best practices. If you are a Performance-minded developers and evangelists then this is a must have tool.
5. YSlow : An Add-on for Firefox that Analyses web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
6. Selenium IDE : An integrated development environment for Selenium scripts that is implemented as a Firefox extension, and allows you to record, edit, and debug tests.
7. Page Ruler : A lightweight ruler to measure web page elements.
8. MeasurIT : Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
9. FireFTP : A powerful FPT client in one tab, your work in another. Sometimes the simplest solutions can be profoundly useful. (Note: Does not as yet support SFTP).
10. FireSSH : A free, cross-platform SSH terminal client for Mozilla Firefox. Written entirely in Javascript!
11. ColorZilla : This extension allows you to pick colors from the web, paste the hex code into other programs, zoom in on pages, measure distances and utilize a built-in pallet browser. A must-have for any web designer.
12. IE Tab : Embedding Internet Explorer in tabs of Mozilla/Firefox
13. Web Developer : The Web Developer extension adds various web developer tools to a browser.
if you feel that some other tool can also be added to this list then kindly comment below. i will update it.