I used to use a Dell Dimension 2400 too, but with only 1 GB RAM. I had a lot of
trouble with the Intel Integrated video when running Ubuntu 8.10, 9.04, 9.10,
and 10.04 LTS. 10.10 ran perfectly; I retired that machine before ever trying
11.04 or 11.10.

My first thought was that you were having trouble with that obsolete video
subsystem. But, upon re-reading your post, I'm more inclined to think that
having that many tabs open in Firefox is crushing your CPU. You might try to
keep a terminal window open, running the "top" command, and watching tasks and
CPU usage as you open up additional tabs. (I've got a still-older and slower
laptop, running Xubuntu 11.10, that will exhibit the behavior you describe with
only three or four tabs open in Firefox.)