By pressing Cmd-D the user is prompted to enter a tag, and after that a .webloc (essentialy a link) file is created in a subfolder determined by the tag name.
The location to store the files in can be configured (i.e. some Dropbox folder, so that bookmarks are automatically shared).
This has the advantage that it's:
* instantly searchable using the same search interface like everything else (Spotlight)
* forwards compatible - no need to import/export bookmarks when switching browsers
* distributed - bookmarks are instantly and automatically distributed (Dropbox)
* automatically backed up forever (Dropbox)
I'm not using Firefox right now, so the extension might be out of date. Maybe time to port it to Chrome/Safari :)
[0] https://github.com/pkamenarsky/fbookmark
[1] https://addons.mozilla.org/en-US/firefox/addon/pentadactyl