If Firefox starts up after an update without your bookmarks and other settings, check whether your data is available in a different profile:

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it. The About Profiles page will open. This page should list at least one profile and could list many.

The profile that Firefox is currently using will show:
This is the profile in use and it cannot be deleted.

Please do not delete anything here; don't use any Remove buttons.

Do you have another profile named default or default-longnumber listed on this page? If so, you can check whether it is the one you were using recently by clicking its Launch profile in new browser button.

If it isn't what you want, simply close that new window.

If it IS what you want, click the Set as default profile button for that profile on the about:profiles page. Firefox will use it automatically at the next startup.

For more information see Recover user data missing after Firefox update.

Note: If your bookmarks are not available in another Firefox profile, see Recover lost or missing Bookmarks for other solutions.

My IT guy at work and I have been trying to use my Default 3 profile. We've been playing this game for 3 weeks now??? It's fine for a day, maybe. Or, 'here and there'. But it keeps saying that "You've opened an old version", "Create a New Profile". AAAAAAAAAnd the problem persists..... day after day. We updated the new browser, created new profiles, re-established bookmarks (my biggest issue for work productivity), yet EVERY FREAKIN DAY, I am dealing with this. About:profiles is a temporary fix. We unpinned the browser, deleted the application, re-downloaded the entire deal... NO SOLUTION. The version will say 144, then 145. Then back to 144 then claim 145 and just BS. Mozilla, I am calling this week. Someone needs to jump on my screen and fix this. I am Fuuuuuuuurious about this update issue. It cripples me DAILY. Do I need to go to Chrome? Microsoft360 is NOT an option but I CANNOT work like this. FIX IT.