Despite the updates this year, my Firefox has been VERY VERY slow on my Windows 7 64, since they did a major update in January, to the point I must restart with addons disabled 95% of the time now, particularly if I go to Facebook. I don't use more than the average amount of addons, either.
I've tried using Javascript editors like "Firebug", but don't really understand how to repair anything, not having the experience, sigh......