Lately my firefox has been freezing up a lot. The mouse still moves but firefox does not respond for a good 5-15 sec. So for example, a youtube video will play and ff will freeze. Then vid stops but audio will continue playing. After a while I regain control and is able to click on the webpage again. I don't think that I'm using that much memory because in task manager I'm only using ~2GB of the 4GB in my system. I've also tried the firefox article on deleting places.sqlite. I also tried the reset button which deleted all my add ons and that didn't help either.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do not click the Reset button on the Safe mode start window or otherwise make changes.