Here is the thread that you started a couple of weeks about that disappearing bookmarks folder issue. https://support.mozilla.org/en-US/questions/1272848

You really should continue in that thread so that other support contributors know the history of this issue.

Are you using Sync to sync bookmarks to other devices ?

You can try to restore an older compressed .jsonlz4 backups in the bookmarkbackups folder.

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore

The name of an automatically created JSON bookmarks backup in the bookmarkbackups folder includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. You also see this count in the Restore menu drop-down list.

• bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4.

You can check for problems with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page

In case errors are reported with "Verify Integrity" close and restart Firefox and retry.

Yes, I am using Sync, and the only other computer it's synced with was off. I did click on the "Verify Integrity" button and closed and restarted Firefox. So now I will see if the problem occurs again.

Thanks

https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

this link will help for you

I have a similar problem. For some reason Firefox on my office system keeps disappearing one specific folder. I always can restore it but for some reason this one specific folder tends to disappear after every update. I have no idea why.