Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

I'm having trouble telling since my bookmarks bar and tools at top are vanishing so I can't get back to my pages easy. Apparently safe mode won't even leave those simple settings.

( One thing I found instantly: I am NOT willing to pay the cost of using FF without adblock. If that is what is causing the slow down either its roll back or switch browsers.

It has been reported that Adblock has problems with the current Firefox. Disabling it for a test will show if this is the problem or not.

Note: Other blocking add-ons are available.

You already have uBlock Origin installed, which is way better than AdBlock Plus in many ways. Running both ad blockers might cause problems, so : removing AdBlock Plus sounds like a good idea

( Unfortunately UBlock is having issues with Facebook. FB is the primary reason for having both

starhorsepax said

( Unfortunately UBlock is having issues with Facebook. FB is the primary reason for having both

Have you checked uBlock's settings where the 'issues' with Facebook are concerned (or contact the add-on's developer)  ? If you keep using both uBlock Origin and AdBlock Plus : you might be stuck with all kinds of problems.

But : this may not at all be related to your current problem - will you please take a look at this article :

Firefox hangs or is not responding - How to fix