I have troubleshot this issue intensively and can reproduce it at will...
With Firefox v4 and BitDefender Internet Security 2011 Antiphising Toolbar v2.0 enabled, the browser stalls and slows, to me, rendering Internet browsing unacceptable.
After disabling the Antiphising Toolbar and restarting Firefox, the browser runs perfectly fine.
(Disabling the toolbar can be achieved by going to the Firefox dropdown menu (top left corner of Firefox) and choosing Add-ons. From there, choose Extensions. Disable Antiphishing Toolbar 2.0)
~Mainly, I am posting this so that the support team is informed. Also, so that the bots can gobble it up and use it to help aid others in finding a short term solution to Firefox 4 running slow, sluggish, jittery while scrolling up or down on web page etc.
I am confident that the fine developers here at BitDefender will find a solution and include it in a future update.
Cheers!