Firefox Profile Downgrade Scenario:
What are profiles?
Firefox saves information such as bookmarks, passwords and user preferences in a set of files called a profile. This profile is stored in a location separate from the Firefox program files. Below is the default location of stored profiles.
Why are profiles causing issues?
Starting with Firefox version 67, Firefox uses a dedicated profile for each installation of Firefox. This makes Firefox more stable when switching between installations on the same computer and also allows you to run different Firefox installations at the same time. Firefox helps profiles keep track of this with the LastVersion= setting of the compatibility.ini file inside of the profile. If the LastVersion= setting in the compatibility.ini file is newer than the installed Firefox, it will “protect” the user from loading that profile in the older version of Firefox. If the source of this change cannot be explained (ie: no actual upgrade occurred), or if you want to force load a profile, you can run firefox.exe with a special flag to bypass downgrade protection and allow the profile to load.
You can make this change by editing the Firefox shortcut and editing the target in the shortcut properties dialogue box for a more permanent method- or you can run firefox.exe -allow-downgrade from the search bar every time.
Managing Profiles in Firefox
If you are having issues loading the Profile you can run firefox.exe -P from the search bar, or inside of Firefox by typing about:profiles in the address bar.
1. Right click on the Firefox shortcut icon. Click Properties.
2. In the Target box, at the end add a space and -allow-downgrade
3. Click Apply.
4. You will be prompted to provide administrator permission, click Continue and enter your credentials.
5. After entering your credentials, click OK in the properties pane.
6. Start Firefox from the edited icon.
7. Firefox should now open without issue.