I am just trying out Waterfox 44.0.2, which is an EXACT clone of Firefox 46.0.1 which I am currently using (Waterfox also updates automatically). When I ran Waterfox the first time, my complete Firefox profile was there (extensions, themes, plugins, bookmarks etc.), but Waterfox seems to load websites much quicker. I sent an email to the Waterfox creator who assures me that support for Waterfox on Mountain Lion shall continue after July 2017, here is his reply on the Waterfox forum:
“I don't see any reason why not. Usually Mozilla end support so if users have issues they don't have to waste resources trying to fix them. Mozilla technically don't support XP 64-Bit, but I've had no issues allowing Waterfox to run on legacy systems. I'll give it a go for you, it's just as easy as changing the version number in info.plist. If anything crops up though it might be difficult to fix”.
So Waterfox 44.0.2 is now my main browser with Safari as my backup. This, of course, is just my experience. I am not much of a techie but Waterfox, as long as it works well, seems to be the answer for me. Ken.