OK, I investigated a bit and see that the problem for me won't be FF4 directly, but the fact that two essential add-ons are not compatible with it right now:
Smart Bookmarks Bar and
Tiny Menu. The first one lets me add bookmark button links to a toolbar while disabling their names (unless I hover over them), which keeps the buttons very small. The second one makes my menus compact.
Looking through the add-ons' comments, a work around will allow me to edit a file for Smart Bookmarks Bar and fool FF4 into thinking it is compatible so it'll work. Tiny Menu needs to be re-worked for FF4 according to the author, and he is not sure if he wants to do it.
I really like that add-on since it lets me fit only my wanted menus on the menu bar. The regular way takes too much room and would make me add a tool bar; I don't want that. We'll see if he updates, or if someone makes a similar add-on.