In OS X 10.6.6 and FF4b9, The very top title bar flickers every time I type a key, or scroll the mouse, or when the bugzilla munching icon is displayed. It did not do this in beta 8.
Specifically, there is a black line between the title bar and the tabs that appears/disappears seemingly randomly and causes really annoying flicker.
For me, it was the whole title bar. But at the moment, it has stopped flickering.
It is flickering again
This may have been fixed already. I installed the Jan 17 nightly build and I don't see it anymore.
Confirmed with 4.0b9 on also MacOS 10.5.8
This is mentioned in the release notes for beta 9 under known issues, Mac OS X. http://www.mozilla.com/en-US/firefox/4.0b9/releasenotes/