If you haven't closed Firefox yet, then you can use "Organize -> Undo" in the Bookmarks Manager (Library; Ctrl+Shift+O).

If you notice you lost bookmarks or accidentally deleted bookmark(s) and you haven't closed Firefox yet then you can use "Organize -> Undo" in the bookmarks manager (Bookmarks -> Show All Bookmarks) to undo this action and restore bookmark(s) you deleted accidentally or otherwise lost/misplaced. You may have to use Undo more than once because Firefox keeps a full undo stack of changes to bookmarks for the current session. Otherwise you can consider to restore the bookmarks from a JSON backup in the bookmarkbackups folder.

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore
• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

This will replace all current bookmarks and you lose bookmarks that were added since that backup was created.

Problem is, the edit option for a bookmark folder is directly below the delete folder option. It is easy to make an error. I deleted my most important folder, 'Office', with all my financial information among other vital things. Separating them would avoid that. Is there any way to alert techs who can do this? I did not even realise for several days so the 'undo delete' was not an option, had I known about it - but didn't.