What they all said. Also, if you have problems, either clear your cache (under history), or hold down the shift key on keyboard and hold it while you hit reload/refresh.
if you've got a 64 bit processor, try Waterfox. It's the 64 bit version of Firefox and seems to run some stuff better than Firefox. I've found that certain video doesn't like to run in FF but runs fine on WF.
When you install it, it takes ALL the FF info and add ons, bookmarks, etc and seamlessly installs then in FF.