You may have dropped the bookmark in the wrong folder.

Did you try to look for it?

If you haven't closed Firefox yet then see if you can use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to undo this action.

What I meant is that the folder disappeared.

If the folder accidentally got hidden from you than do the following:

• Go to view options of my computer.
• Open to the folder options.
• Go to view.
• Uncheck all the hidden options. (as shown in the picture)
• Press ok.

Now see that if the folder appeared. HOPE IT WORKS :)

Are you using Sync?

You can check for problems with the places.sqlite database file in the Firefox profile folder.

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

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

Note that you can move folders as well.