For those using Firefox 4.X, 5.X, or 6.X

[Head]

I've stumbled on to a bug with Firefox 4.X and 5.0 that may affect you depending on your operating system and video drivers, possibly.  Basically, if you look at a page with a lot of animated gifs (those smiley things), it will cause Firefox to use a lot of CPU, up to 100% which makes doing anything impossible.   

Workaround: in the address bar type about:config and in the filter box, type in anim and look for image.animation_mode. Right-click on it, choose modify and change the value to either "once" to play the animation once or "none" to stop them completely.

I've posted about in the mozillaZine forum: http://forums.mozillazine.org/viewtopic.php?f=38&t=2234609 and there's a bug about it: https://bugzilla.mozilla.org/show_bug.cgi?id=595671

Firefox 3.X has no problems.