I've been using FF for many years going back to the time when they had tabs before IE copied them. These days I can't (as area51drone mentioned) live without all the add-ons I've grown used to. I have grown more and more annoyed at their seemingly random and capricious user interface changes over the years. I mean how many times have they changed the way you clear cache / delete cookies and such?
I have tried to avoid upgrades because last time I did it broke one of my favorite add-ons that never got upgraded (some toolbar gadget). Anyway I was on like version 17 or something for so long, but recently, Yahoo Mail, which has imho completely totally screwed up their once simple, fast, useful inteface (different story) kept demanding that I upgrade to the latest FF and after not being able to turn it off I finally gave in and upgraded FF to v25. I think they must have jumped several numbers in there but whatever.
Now, what did I get? Not only another huge interface change, including burying what used to be the Enable Javascript checkbox in the friggin about:config settings because they felt it would "provide better user experience" or some junk like that. But they friggin changed the resolution rendering to match whatever you have your windows DPI settings at, which mine are at 125% because I can't read text the size of a peppercorn.
So, in one fell swoop, FF now has managed to pretty much hose up my entire web browsing experience. It pretty much broke every site I frequent including this one by making them render way too friggin big with way too much white space and messing up all the fonts. It took me like a week or two to patch things up to I can sort of read things again but it's still basically broken.
I used to really really love FireFox, and I'm still hooked on their damn add-ons, but jeez I'm feeling the hate more and more these days.