Just recently I tried the FF 4 "Minefield" to check out the 2D GPU rendering. It said it was enabled but I couldn't tell. Probably because I don't have game grade GPUs. Nowadays I mainly keep FF for sites Chrome won't handle and also to fix up loss of favicons and that type of thing in Chrome. LastPass will work in both but there doesn't seem to be a simple way to cross sync bookmarks that doesn't produce dupes and other side-effects.
But back to the glass.. the main thing that gripes me is when they try to get a large glass caption bar, a lot of apps, including Chrome, want to use a ribbon instead of the standard menu bar. So now everything is crammed into one drop-down menu item. It's a pita. I hope Windows8 adds easy to use APIs to draw text and graphics on glass without jumping through flaming hoops.