1. ColorZilla – A collection of tools which can assist with a variety of colour related tasks.  The ColorZilla tool that we use frewquently is the ‘Eyedropper’ which allows you to take a sample of any colour that is currently visible in FireFox.  ColorZilla then allows you to copy that colour sample to your clipboard as an RBG or HEX reference.  The Eyedropper comes in highly useful when working on a website or design and you need to quickly find out what colour is currently being used on the website.
2. Measureit - Basically a ruler for your browser, Measureit allows you to quickly measure areas on screen within FireFox.  Area dimensions are given in pixels meaning it’s an invaluable tool when working on a website or design.
3. Xmarks – Store all your bookmarks and passwords in the ‘Cloud’ with Xmarks, the FireFox synchronisation tool.  Xmarks works by syncing all your FireFox bookmarks and login details with your own personalised account, so as long as you have Xmarks installed, anytime you use FireFox on any computer it will have all your details synced.  Xmarks is highly useful is you work from more than one location or computer; there is no worse feeling than working out of the office and finding you don’t have all your saved bookmarks or saved login details.
4. Server Spy – Apache?  IIS?  Zeus?  ServerSpy allows you to quickly find out what server technology and version a website is running on, very useful for looking at new client websites when suggesting web platforms, or debugging website problems.
5. FireBug – To be honest, I was going to leave FireBug out of this list since it’s been talked about so much, and if you’ve not come across it by now all I can really say is go get it!  FireBug takes the headaches out of debugging live CSS or HTML, and allows you to make changes to the website code in real-time.

Image by Foomandoonian