as a designer i use the ones i mentioned at my blog http://www.designussion.com/12-firefox-addons-that-make-a-web-designers-life-easier/
I use the google tool bar, Brief (makes reading RSS feeds as easy and intuitive as it gets), multiple tab handler, adblock plus, google preview, download status bar, WOT, and Targeted Advertising Cookie Opt-Out (TACO).
I have the Validator plugin, Firebug which is awesome and one that I found that saves the page as a img / jpg rather than screenshotting
Very best addons for me are: 1. Roboform addon ( save passwords cards) 2. Noscript addon ( block java) 3. Flashblock addon (block flash with CPU-intensive) 4. Imacros addon (useful to automate everyday deals such as postinf informations to classified, job actualization etc.) 5. IE View 1.3.7
NoDoFollow, AdBlock Plus, Skype, PPCWebSpy and Tamper Data (do not use the last one anymore). Already have autofill so can't be bothered to get RoboFill.
I use Maxthon as my daily browser, but I love Firefox plugins. I use Firebug (of course) and also Rank Checker. And Roboform, as someone mentioned.
adblock cooliris tab close button (my all time favorite add-on) slimsearch adblock I also use tab wheel scroll - it takes some getting used to, but it's really convenient to just scroll through all the tabs by moving the cursor on a tab.
I'm using the Dofollow-Nofollow add-on. It helps me to identify blogs which allow dofollow comments. Thus it helps in SEO.
i use twitterfox to update my twitter without logging in to my twitter account and search status to know the alexa and page rank of sites i use these 2 addons in my firefox browser Good luck
Abduction, Google Toolbar, Spellbound, Firebug, Seoquake, Dictionary, Password Exporter & Web Developer. GO FIREFOX