(IIRC, I also noticed this in a few of the later Firefox2 builds...)
Anyways, two issues I’ve found while using MODx CMS in Firefox3 (Beta 4)...
Tree Navigation scroll bar partially works --- meaning the right side is completely unresponsive.
Hide the Tree. Then try clicking any of the "buttons" in the top frame. They might work, but only for a second, thus are unresponsive unless you un-hide the tree. I had noticed/reported this issue in some of the early Opera 9.5 Alpha week;y vbuilds, although that fixed itself eventually. But since Firefox3 is essentially close to RTM (only one more Beta, IIRC), this isn’t a good thing.
The scrollbar in the tree is a css issue; there’s a fix for it in the forums (of course I can’t find it right now...)
It seems the "button" issue is also CSS-related. The "top" half of the button responds, but not the bottom half. Kinda like the left-right issue with the scrollbar.
☆ A M B ☆