The Firefox add-ons are great especially for a web designer, very seductive. Firefox is just too slow and too insecure for me personally and I couldn't let my users use it as it is again too insecure (still no split-privilege, low integrity whatever it's called) and is non-configurable by group policy as far as I'm aware, unless you perhaps could do some group policy extension hackery.
Apologies if this view is very out of date, which is highly likely, Firefox may very well have improved since I last looked at it.