my firefox experience was plagued with performance problems for quite a few releases, including 4.0. a big problem was that every few seconds it would pause for a moment which made scrolling and watching videos unbearable. (googling "firefox freeze" shows a lot of people have this issue.) i tried uninstalling all of my extensions and installing them one-by-one to figure out what caused it, but the problem never got significantly better.

i was about ready to jump ship to another browser until i decided to completely wipe out my firefox profile and start over. after doing this, my performance problems have not come back, even after manually re-installing all of my add-ons, greasemonkey scripts, etc. i don't know what kind of cruft my firefox profile had accumulated over the years (i've even migrated it between operating systems), but starting over fresh solved a lot of problems for me.