Sorry to hear about that, it sounds as though the Refresh did not complete.

You can create a new Firefox profile to get up and running -- it won't have any of your data -- and then copy old data into it.

To create a new profile, please see this article: Profile Manager - Create, remove or switch Firefox profiles.

After you get Firefox running in a new profile, could you check the following two places for old data:

(1) Desktop

A Refresh normally moves your old settings (profile) folder into a folder named Old Firefox Data. Can you find that?

(2) Profiles folder

Open your new uncustomized Firefox settings (profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show in Finder" button. This should launch a new window listing various files and folders.

Click "up" a level in Finder to the Profiles folder. Do you see any other profile folders here other than the new one you just created?

Hopefully with a folder from one of those two places we can get you back your most important data (e.g., bookmarks, saved passwords).

I've got Firefox back now, and I found the "Old Firefox Data" folder, but I have no clue how to get it into the new version of the program. From the Firefox Support pages I was linked to I can't comprehend how to make it work.

Also, the original problem that caused me to start all this — garbled pages in Google Sheets — still persists.

It depends on what you want back, just selective data, or everything.

For selective data, you can copy individual files from the old folder to the new folder. This article describes what's in there: Recovering important data from an old profile.

For a wholesale "roll back" I suggest the process outlined in my post in this thread: https://support.mozilla.org/questions/1080782#answer-774455

Regarding Google Sheets, could you describe the "garbled" problem in more detail?

Also, does it make any difference if you try in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the option key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)