T.T I'm not sure why...and it seems to be taking up way too much memory (700k)... I would just switch to opera but a ton of stuff of sites aren't supported by opera. :S
It seems to happen to me as well sometimes, I used to think it was because I had too many tabs open, but I'm starting to think it's due to certain sites... Hotmail seems to cause some lag for me sometimes but only if I have other tabs open.
restarting...doesn't really help (a little but not much) cache thing...maybe? but I erase private data every time I log into a new account addons...most likely not (I have only a few) too many tabs...maybe? I often have like 100+ tabs open at once (plus I close FF very rarely...and restart my computer even less) but it's laggy even when I have like 1 or 2 tabs open too :S