MAC's Mac is left on overnight. Come morning, MAC has lost power due to bad UPS I rebuild bookmarks and Add Ons. . Power up, & Rebooted and Firefox comes back as a new install with no bookmarks and no ADD ONs.
This has happened several times.
Go to your local department/electronics store and look around for a UPS (Uninterruptible Power Supply) that is useful for you. The lowest price is $50 +. There are also units on the market that plug into the computer. So if the battery gets low, it will shut down the computer to protect it.
Note: Sudden power loss can damage or destroy a hard drive's data.
Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.
Type about:profiles<enter> in the address bar. How many profiles are listed? How many should be there? Also, open the profile folder in your file explorer.