I've only been using Firefox since Sunday and haven't installed any extentions yet, but I'm satisfied with it's pop-up blocking abilities. I don't think Firefox is any better than Safari because I'm still seeing the spinning beach ball of death a lot, some sites aren't rendering properly, I can't easily close tabs or open a link in a new tab, and I still haven't figured out the bookmarks yet. These minor Firefox annoyances are due to Safari being specifically designed for the Mac and it's one mouse button, but I can get over these things. I have to get over these things because I can't stand pop-up windows!
Update
PithHelmet is working for me now, so I'm using Safari again. Firefox kept freezing up and crashing.
