More info. Firefox starts (using SAME profile as always), but bookmarks are empty? In the active profile, there's folder "bookmarkbackups." Can restore latest copy.

In bookmarks library > Restore > choose a json file from bookmarkbackups (any recently used profile where "full" set of bookmarks were used).

If done a bookmark BU to HTML file, that's for the "import bookmarks from html file" (or if did a recent system or just Fx profile BU, can recover them from there.).

See also:

• https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update
• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks