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

You can open the about:profiles page via the location bar to check whether multiple profiles are present and what profile Firefox currently is using.

• Do NOT click any Remove button on this page.
• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Firefox 67 and later can use a dedicated profile for each of the Firefox update channels and will lock the profile, so it can only be used by Firefox in a specific installation folder. 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
• https://blog.mozilla.org/2019/01/14/moving-to-a-profile-per-install-architecture/

<deleted> I see cor-el already posted a similar answer. P.S. Here is the answer I deleted, since I see it was marked as the solution:

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

Open the Profile Manager page:

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

This page should list at least one profile and could list many. Each will show links to a pair of folders; please ignore those for now.

The profile that Firefox is currently using will have this:

This is the profile in use and it cannot be deleted.

Actually, 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, back on the about:profiles page, click the Set as default profile button for that profile so Firefox uses it automatically at the next startup.

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

Thank you! Problem solved. I had to set my old profile as my default. All is good :)

Ubik_Montana said

Thank you! Problem solved. I had to set my old profile as my default. All is good :)

Great! Thanks for posting back with the outcome.