Old versions at Filehippo
No need to uninstall usually. Just double click the new (old) setup file.
Should you wish to delve further and possibly use Revo, Mozbackup can backup various categories of settings of your current profiles first.
Easy to restore if anything goes wrong.
Personally, I have thought about changing broswer. FF is a tad sluggish for me (but not too bad).
But I really like all my add-ons and how it is setup. Not all add-ons are available or compatible on other browsers. I'll probably stay put for a while.