In reply to pec:
You can do a system restore to a date just before FF was updated to Quantum and that will get you back the old browser, then you just have to turn off auto updating before rebooting your computer to stop it from doing the updating all over again.
Just keep in mind that any system changes you did in that period will also be rolled back, and that the earlier Firefox will stop receiving support in the form of security updates and stuff soon, so you may end up having to change browser anyway.
I also hate quantum and have had a play with Waterfox, which is based on firefox, and a lot of the settings and stuff from firefox get migrated, so you can have a play and see if you like it before uninstalling FF. FF and WF can't be open at the same time though, so if you try it be sure to close FF before opening WF
Post edited at 22:49