My firefox is currently overwhelmed by the number of toolbars I need for my day to day work. I actually switch browsers for personal tasks, since firefox is so weighted down. Firebug - Debugging Javascript, and trying to emulate sign-ups to complex sites Social Bookmarking(various) - God I hate those. RefSpoof - Cloaking testing Google Toolbar - Bleh. Occasionally uninstalled to keep the big goog out of my business. SeoQuake - I love it and hate it. Probably the single largest stability issue on my entire system, but very useful.
I am currently using Google and Stumbleupon toolbar. No plans to add any other because it will clutter up my browser.