No, I noticed the same thing.
Firefox 3.5 still has some bugs in it. Mine ran slowly too, and even crashed. I had to do a control alt delete -then end firefox.exe process. Then restart firefox with a new session. After a few hours on it would begin to load pages slowly.
I uninstalled firefox 3.5 from the control panel (add and remove). Then I downloaded firefox 3.0.10 from Oldversion.com. 3.0.10 can still be updated and works better than 3.5 for me.
Here's the download if anyone needs it.
http://www.oldapps.com/firefox.php