Not possible.

Just right-click on what you want to delete.

Note that Firefox keeps an undo stack for all changes made to bookmarks in the current session.

If you notice you lost bookmarks or accidentally removed a bookmark and a search in the Library doesn't find them and you haven't closed Firefox then try "Organize > Undo" in the bookmarks manager (Bookmarks -> Show All Bookmarks) to undo this action and restore bookmark(s) you deleted accidentally. You may have to use Undo more than once (Firefox keeps a full undo history). 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.