Look on your desktop. Do you see a folder called; Old Firefox? Look inside.
Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

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.

Which of the two profiles are you using as your current profile folder ?

Is that the default-release profile ?

Are there compressed .jsonlz4 backups in the bookmarkbackups folder in either profile that can be used to restore bookmarks ?

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore

If you lost personal data then it is possible that a new default-release profile folder has been created. See "Determine if Firefox has created a new profile":

• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

Firefox 67 and later can use a dedicated profile for each of the Firefox update channels.
For Release versions this profile would be a xxxxxxxx.default-release profile, profiles used in older Firefox versions usually used a xxxxxxxx.default profile (a refresh adds a time stamp).

• https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation

Note that with the default profile in the third screenshot there have been multiple issues with saving prefs.js as there are a lot of numbered temp prefs-##.js files.
You can check the file dates to see when this happened.

This problem with Firefox updates has been there for years. I've had this issue 5 years ago and I can't believe I'm having this issue again in 2020! It seems Firefox needs to look into this issue seriously so we don't just keep on doing it manually. I had to find the saved profile elsewhere then get to about:profiles in the browser and install the saved profile as new profile. This may seem a simple fix but doing it is a big pain specially finding the whereabouts of the automatically saved profile and surely it isn't for beginners or the unknowing.

Hi,

The people who answer questions here, for the most part, are other users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.

You can also file a bug report or feature request for Firefox and other Mozilla products. See File a bug report or feature request for Mozilla products for details.

Back up and restore information in Firefox profiles {web link} I use an add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.