An apparently fresh Firefox window after an update does not prove that your bookmarks have been erased. Mozilla explains that different installations can use different profiles. The first task is to identify the profile containing your work, while preserving all candidates.

Compare profiles without removing them

Type about:profiles into Firefox’s address bar and open the page. Note which entry is marked as the profile currently in use. If another entry exists, use Launch profile in new browser to inspect it in a separate window. Leave every Remove button alone.

Look for several distinctive bookmarks or folders rather than one familiar page. Check whether the profile contains recent work as well as older material. A profile belonging to another household member may look plausible but be the wrong recovery target.

Make the recovered choice deliberate

If the other window contains the missing information, record its profile name before choosing it as the default. Close Firefox normally, reopen it and confirm that the expected profile appears. Avoid making new bookmarks in several windows while deciding which one to use.

If none of the listed entries contains the data, stop before creating a series of empty profiles or reinstalling again. A profile folder can remain on disk even when it is no longer listed. Preserve existing folders and follow Mozilla’s further recovery instructions, or ask for help identifying them.

Once the correct profile is established, arrange a separate backup and test a small export. The result to record is which information was recovered and from which profile. Finding bookmarks alone does not confirm that every saved login or open tab returned.

Source: Mozilla Support.