You can simply reload 3.6 over 6, and it will happily revert. That's what I did, as I dislike intensely the new appearance - transparent, aero, whatever - and the inability to get rid of the features simply, if atall. The problem is it will be less secure as time goes on.

I just switched to Pale Moon, a FF variant, and got most of the addons, plugins working fine. If the people at FF weren't so arrogant as to say, in effect, this is what you WILL now have, no argument, I may have stopped longer, since v3.6 was fine.