If you haven't already done it, I recommend installing a flash blocker add-on for Firefox here.
With a lot of tabs open, and flash running, Firefox can really be hammered by some sites. Most flash animations are fine, and I haven't figured out why some are a problem. But anyway, having ads bounce around in front of you when you are trying to read doesn't make for a pleasant experience.
The blocker turns every flash animation on the page into a play button, a bit like Youtube. When you click play, it plays the flash animation. You can also tell it to allow flash on certain sites (such as Youtube).