in your description there is one point that makes your request that firefox should always do the right thing _without_ manual intervention void: you say "set View|Character Encoding to UTF-8" ... which is clearly a manual action and once you did that there is no real reason why firefox should not honour what you set. So, as long as you don't leave the no-manual action path everything will be fine. ONce you change it manual its ok to assume that you find that feature again to set it to auto again.
in your description there is one point that makes your request that firefox should always do the right thing _without_ manual intervention void: you say "set View|Character Encoding to UTF-8" ... which is clearly a manual action and once you did that there is no real reason why firefox should not honour what you set. So, as long as you don't leave the no-manual action path everything will be fine. ONce you change it manual its ok to assume that you find that feature again to set it to auto again.