When more than two tabs are open (no matter the content), FireFox (3.6 and 4.0) slows down and the CPU usage value on the Task Manager shoots up to 100%.
The Memory usage also shoots up to about 475k MB. This has happened over two computers, the first being a stock Dell Inspiron 1620 with Windows XP and this one being a stock HP G72 with Windows 7. I used a data transfer cable to transfer all settings from the first computer to this computer. The problem persisted through 2 updates (3.5>3.6>4.0). The only FF add-ons I have downloaded are NoScript and AdBlocker; I also have several unused toolbars downloaded from various other applications (Yahoo, Norton, and AVG) and frequently use the Google toolbar, which in an unrelated matter stopped saving my searches or suggesting searches while typing at the 4.0 upgrade. If you need any more information, I'd be happy to post it.
Opaite Mbohovái (2)
Try running Firefox in Diagnose Firefox issues using Troubleshoot Mode. If it functions properly in that configuration, then one of your add-ons is the culprit.
Try a new Profile in Firefox 4 and see how it is.
http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows