EDIT: The bookmark manager I pay for is called Linkman - http://www.outertech.com/en/bookmark-manager. I first bought it around 10 years ago and still use an old version, so unfortunately I'm neither up-to-date with its present capabilities nor the current landscape of 3rd party bookmark managers.
I'd pay money for a book mark manager that could organise my list of about 2,000 bookmarks. That software could have a "bulk bookmark import" feature so that users can share some set of bookmarks. "Here is my list of best pages about $ANIME_SERIES".
A bookmark manager that looked a bit like Pinterest, and that has options to locally save some content, is something I'd pay money for.
One solution would be to let you install third-party bookmark managers which would replace the built-in bookmark UI. That seems even more ripe for abuse than features like custom toolbars, though, which are already heavily abused in current browsers.
give me all bookmarks that are tagged "tools" AND "unix" AND "visualization" AND (NOT HAVING "monitoring")
There is an add-on called "tagsieve" that I used heavily, it had a nice visual possibility to filter the bookmarks with a tag cloud - though it didn't have a search language. But throughout the years often it was not compatible with new Firefox version (just see that it is installable again).