I've frequently installed several versions of mozilla - stick one in ~/home/bin with the other in /usr/bin or wherever it is, and so on. So you don't *have* to uninstall. Your bookmarks are in ~/.mozilla/blah/blah/bookmarks.html but that part won't get uninstalled either way. Still a good idea to have a backup - just 'cp' it somewhere else.
Still, it is safest to uninstall the old, I suppose. Nothing special I can think to warn you about, though. Go for it.