Are you sure it disappeared? Could it have been moved? Or renamed?
Firefox makes backup copies you might be able to open.

Recover lost or missing Bookmarks

Restore bookmarks from backup or move them to another computer


I would also recommend;

FEBE (Firefox Environment Backup Extension)

FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files.
It will also make backup of files that you choose.

Note that if you notice this before having closed Firefox that you can use Organize > Undo in the Bookmarks Manager (Library).
Organize is the first of the three buttons on the toolbar in the Library (Bookmarks > Show All Bookmarks).

even if i do a search for the folder on all files on my mac, I can't find it. I restore bookmarks, and that one folder has disappeared.

Just to clarify. It's a whole subfolder of bookmarks that has disappeared.

How come even if I click "RESTORE", to the last date I am sure I last saw it, even to earlier dates, that subfolder has totally disappeared. Help!

Did you try to use the search bar to search for a known lost bookmark in case you moved this folder into another folder?

Note the the current Firefox releases show the number of bookmarks in a JSON backup as part of the file name (xxxx in bookmarks-####-##-##_xxxx.json), so you can look for the file with the most bookmarks.

If you use the Time Machine on your Mac then you can restore an older version of the places.sqlite file.

• http://www.apple.com/support/timemachine/

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)