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.

14. HTML Validator: Displays the number of found errors of the site you are viewing. It adds HTML validation inside Firefox and Mozilla.

if you feel that some other tool can also be added to this list then kindly comment below. i will update it.