In the Desktop Firefox "Add Bookmark" / "Edit Bookmark" window (Ctrl+d), after expanding the "Location" part - showing the Bookmarks Folders tree - I suggest adding a fast search line under the folders tree, using the same fast search engine than the "Ctrl+f" feature (allowing fast text search on the whole page by simply typing some letters of a searched word/phrase, without even pressing ENTER), composed simply of a text input line and a next/previous buttons.
Adding fast search line would work best with opening the folders tree already fully expanded (folders and all levels of subfolders) for default (without necessity of changing the usual tree functionality).
Maybe it could work better and could be easier to implement if the "Add Bookmark" window would be a "normal window" with standard maximize/minimize/close/resize/move features - in place of being only a popup window.
Now adding a new bookmark to a specific folder is usually a complicated process, needing scrolling the folders tree and/or expanding some folders and subfolders (even much more complicated because of a small size of this window and folder tree box and by lack of possibility of resizing this window) in order to find the desired subfolder. Adding a fast search line could improve this dramatically.
Similiar but not the same already posted ideas about adding some search possibility here: