just stick with an older version so FF, like v 49 for example and turn off all updates. btw, a really useful FF extension is checkcompatibility that lets you add back disabling compatibility checking so you can use old extensions.
Isn't there a concern about the lack of security fixes sticking with an older version of a browser that isn't updated? Even if that wasn't a problem, I don't know how far back I would want to go. I can't remember when I started having memory leaks/usage issues with Firefox.