A few times in the past I also had this issue, and it was caused by a problem with my browser profile.
Sometimes, a recently added bookmark or recently opened tab would also disappear.
If you go to about:support, look for the Places Database section and click Verify Integrity, does it report any errors?
I advise to first export the bookmarks, because if it’s really that, the above procedure may result in more lost bookmarks (it makes a backup automatically, places.sqlite.corrupt in the profile directory, but better safe than sorry).
The cause of trouble, in my case, was due to profile-settings-daemon taking too long to sync the profile back to disk and getting killed by systemd on shutdown.
That should only happen on slow machines like mine though.
See also Fix PSD (profile-sync-daemon occasionally) failing to sync the browser profile due to a rather short systemd default timeout
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