1. Web Developer
2. FireBug
3. FirePHP
4. ShowIP
5. YSlow
6. Febe
7. FireFTP
8. HackBar
9. User Agent Switcher
10. LocationBar
11. LiveHttpHeaders
12. QuickLocaleSwitcher
13. Flash Downloader
14. ColorZilla
15. SpeedBar
16. SpeedDial
17. FireQuery
18. FireCookie
19. ViewSourceChart
20. MeasureIt
21. British English Dictionary
22. DownloadHelper
23. IMacros For Firefox
24. Delicious Bookmarks
25. KPMRS
26. Dust-Me selectors
27. Aardvark
28. Fangs

Other Software for development
1. Web accessibility toolbar
2. IE Tester
3. Windows magnifier
4. Xenu’s Link Sleuth