Firefox has protection in place to prevent you from downgrading. The reason is that sometimes how Firefox stores your data (bookmarks, etc) changes during an update. If you were to downgrade back to a lower version, you risk Firefox not being able to read your data that has been adjusted to the new format. So Firefox makes you a new blank profile when you try to downgrade.

That said, you can still try to force Firefox to allow you to downgrade to a previous version and it often doesn't cause any issue.

Of course though, you will want to back up your Firefox profile(s) BEFORE attempting to downgrade Firefox, so that you have a copy of the data in the event that downgrading Firefox doesn't work.

To force Firefox to allow you to downgrade, you need to use the -allow-downgrade command line option when you start Firefox. You may also need to use the -ProfileManager option as well to allow you to select the old profile.

An alternative (and potentially easier) option would be to upgrade back to Firefox you had before and then using Firefox Sync to move your bookmarks and other data to the downgraded version 75. See How do I set up Sync on my computer?

Hope this helps.

To preserve your currently available data, please make a backup copy of the following folder in a safe place.

C:\Users\username\AppData\Roaming\Mozilla\Firefox

Since AppData is a hidden folder, you can use this shortcut to jump into the Mozilla folder:

%APPDATA%\Mozilla

If you paste that into the Windows 10 search box, it should recognize it as a folder and let you click the File Explorer icon to open it.

Thank you very much!