If this ever happens, you should be able to open the Bookmarks / Library window and select Organize > Undo. (The regular View >Undo option in the main window serves other purposes, so that won’t do it.)

For accessing the Bookmarks Manager / Library, see the Bookmarks in Firefox article.

I was not able to recover all of the bookmarks originally deleted.

If you haven't closed Firefox then try to use "Organize > Undo" in the bookmarks manager (Bookmarks -> Show All Bookmarks) to undo this action and restore bookmark(s) you deleted accidentally. Note that you may have to use Undo more than once. Otherwise you can consider to restore the bookmarks from a JSON backup in the bookmarkbackups folder.

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

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

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.

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