Because I lost some of the adons I regularly use I went back to version 56.0b9 (Quantum is version 57).
Install an older version of Firefox notes.
I found that I didn’t have to uninstall version 57, I just installed the version I wanted and it took Firefox back to version 56.0b9 without any problems.
If you want to go back to an earlier version be sure to disable automatic updates.
(Menu bar / Tools / Options, on the General tab scroll down to near the bottom of the page and you’ll see the update options - Allow Firefox to)
If you use Firefox Sync like I do it will make sure your adons are installed to 