Most useful firefox extensions are...

1. Firebug 1.10.6 for developing help in code.
2. Google translator for Firefox 2.1.0.3
3. IDM CC 7.3.41 for internet download manager
4. Pearl Cresecent  Page Saver Basic 2.8 to take a snap of full web page
5. PixelZoomer 1.3.1 to get div,image height and width
6. Test Pilot 1.2.2 to do Firefox better 
7. Web2PSF converter 1.12 to create web page pdf