When saving a bookmark, Firefox provides a small dialog that allows you to select a folder in which to save the bookmark as well as adding some tags. If you've got a lot of folders, including nested folders, it can be difficult to find the correct one using this dialog.
What I propose is:
(1). Add a "search" box at the top of the dialog. User enters a word (or a few words), and FF searches the names of all folders, and the URL and page titles of all bookmarks in each folder, ranks the quality of hits, and offers a short list of the existing folders that seem like the most likely place to file said new bookmark. If user doesn't like the suggestions, they can ignore and use the folder selection process as it currently exists.
(2) A more intelligent version of (1). Same idea, except user does not need to enter a search term. Firefox takes the URL and the title of the page that is being bookmarked and searches against all existing folder names and bookmarks, and then offers a short list of the existing folders that seem like the most likely place to file said new bookmark.
(2a) Same as (2), except Firefox also extracts keywords from the text of the webpage that is being saved, and uses those keywords to identify the best existing folders. Only worthwhile, of course, if (2) alone doesn't have enough information to give good recommendations.
(3) Autogenerate tags. Firefox analyzes the page title and page text, extracts keywords, ranks keywords, and pre-populates the "tags" field in the Save Bookmark dialog using top ranked keywords.