It's a little too easy to drag and drop a bookmark folder into another folder, and then it's a little hard to find, but it's usually only one level deep wherever it landed. I suggest using the left pane of the Library window to explore. You'll see a little triangle to the left of each folder on the Bookmarks menu so you can look inside at the next level to see whether the missing folder shows up there. Either:

• Ctrl+Shift+b
• Bookmarks menu > Show All Bookmarks

Did that turn it up?

Oh, maybe this will be faster. Click the blue star, then on the far right side of the "Folder" line, click the button with the symbol like a wide "v" to expand the full folder tree. If you're lucky, Firefox will be highlighting the current folder in its exact location in the hierarchy.

After several restarts, the folder has appeared.

Are you using Sync to sync bookmarks?

When this happens again then do a check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• https://addons.mozilla.org/firefox/addon/places-maintenance/

See also:

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file