Firefox warns you it needs restarted, so unless you really hold the browser open for days on end (just how much RAM do you have?) that is dealt with.
Also any new instance of a call (e.g. starting flash for a new video) gets the new version, so unless you are watching the same compromised video for weeks, same applies.
Its not perfect, but it seems a better approach than Windows where you cant replace an open file, so all sorts of stuff has to be done on shutdown/restart.