I finally found a thread stating how to fix Firefox and Flash. It comes down to adding "XLIB_SKIP_ARGB_VISUALS=1" in front of your firefox command - I typically just modify the menu item.
This is due to the new composite extension being enabled by default in FC5. Another thing to note: I need this exact same option to make gnucash work for me. I didn't see the connection before since gnucash would fail to even start, whereas firefox worked, it just crashed on Flash pages.