My favorite version of Firefox is the Palemoon project.. ..its also Open Source, and you can check it out
here
really, I don't know why it's not more known ..it's a really good version, more optimized, they keep it up to date, and without the mood changes from random updates (I really stopped worrying about if I should or not update, like I did on standard firefox.. ..now I can always update without the danger of crazy surprises).
I know my message turned out looking like a ad haha ..But really, I like it and never had big issues with it on the last years
Has someone else here already tried it too? ..maybe the OP?