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.

Hi, Paul, thank you so much for taking the time to reply.

Alas, when I open the "Profile: default-release", my old Default profile data doesn't show... "Funnily" enough, Firefox kept the name of my "old" Default profile, despite erasing all data from it....

Any other tips are much appreciated. I checked the links you sent before posting my question, but they all mention old files I cannot find, and "recover" options that I'm simply not given...

Could I get help with my problem, please? I really don't wanna believe all my bookmarks and history is simply gone, because of an update..

That sounds really frustrating. Sometimes after an update Firefox can’t properly load the old profile and just creates a new one, which makes everything look gone. I’d check the profile folders carefully — your old data might still be in a backup or sessionstore folder. If anything’s there, you can usually restore bookmarks and history manually. It’s stressful, but there’s a chance some of your data is still recoverable.