How can I downgrade Firefox?

There is no built-in rollback or downgrade function within Firefox. In fact, on their support page for the subject, they strongly insist that you do not roll back.

I rolled back a longstanding, many times upgraded, Firefox 57 to the specific version and bit depth below. I did not manually uninstall Firefox, but was prepared for things to go sideways. YMMV.

If you know better and don't want to heed their warnings, then do these steps:

1. Download an older version of Firefox here. If you are trying to back down from 57, you'll want to grab 56.0.2, (Windows US English version 32 bit exe) here. You can look at all of their 56.0.2 releases here.
2. Wait until the download finishes.
3. Find your profile folder. You can find where that is here. Also, this Mozilla Support article covers steps 3, 4, and 5 of this answer.
4. Close Firefox.
5. Back up your profile folder. You can simply copy the folder somewhere else, just in case stuff goes sideways.
6. Find the executable you downloaded in step 1.
7. Run it.
8. Go through the installer, and when its complete, launch Firefox.
9. My previous session, extensions, and settings were all as they were before the upgrade/downgrade. I was pleasantly surprised.

If your install is broken, try the usual steps to fix installation problems:

1. Completely uninstall Firefox via Programs and Features
2. Reboot
3. Run a cleanup utility like CCleaner.
4. Manually remove the leftovers and profile folder if you have to.
5. Install from the 56.0.2 executable you downloaded.
6. Verify it works with a new profile.
7. Restore your backed up profile (Mozilla Support)

See also: gHacks, MozillaZine.

I didn't roll back my 16.10 installation of Firefox 57, but installed Firefox-ESR alongside it.

https://launchpad.net/~mozillateam/+archive/ubuntu/ppa

all my extensions seem to work fine

better yet.. https://www.mozilla.org/en-US/firefox/organizations/all/