Firefox 4 does have some minor glitches.
My open windows (next to the Start Button in XP) sometimes do not always show all the open browser windows, even though they are in fact, still open.
Sometimes, Firefox 4 resets the position of my buttons on the URL bar and reinstates the default Firefox search box, which I had told Google Toolbar to remove and replace with Google Search box. I am not sure if this is a Firefox issue or an issue with Google Toolbar Version 7.1.20101113Wb1
Once in a while, I shut Firefox from the X in upper right and yet in Taskmanager it is still running. Other than these minor bugs, it seems ok so far.