One such example I hit daily is the Slashdot comments. Take this page
for example. If you scroll down the page it becomes really slow, but it only happens when the floating div with the number of comments etc is following you. If you click the little button in the top right twice to embed it in the top of the page, scrolling is back to full speed. So it seems to be that performance is affected by certain divs or perhaps the way firefox 3 is drawing that pixmap changed.
One other example of slowdown that's easily reproducible (it was posted on these forums before, but it may of been lost) is XFCE's default terminal, "terminal". If you try to maximize it it can take up to 2 seconds (minimizing is a lot faster, but still has a small delay). Perhaps this will help determine the problem as the XFCE terminal application is a lot more basic than FF3.
Other than those two examples (and a few other web pages), my general 2D performance on my 8800GTS G92 is fine (using XFCE as my DE).