One feature I've wanted since forever is a full text search engine of only stuff I've previously seen before. I've spent hours typing in various search queries for an article I'm sure I read 3 years ago but am fuzzy on the details of. Doesn't seem too hard to do technologically and would solve a real problem in my life.

The Firefox address bar doesn't let you tag things by keyword afaik, but it is pretty good at pulling up articles based on both their URL and title. It's definitely improved my ability to go back and find stuff from a few weeks or months back.

You definitely can tag bookmarks in Firefox, and typing the tag will bring up that item.

Adding a tag to bookmark: https://i.imgur.com/iVElOWb.png

Searching by tag: https://i.imgur.com/e3UmQDG.png

I don't use any bookmark management addons, so this is definitely default behaviour.

Perhaps session buddy, if you're the type to have a bunch of tabs on a topic. You can search for names of sessions, or for pages in a saved session.

Something like Pinboard?

https://pinboard.in/

Pinboard is a bookmarking solution, not a middle ground between bookmarks and browser history.

I am currently evaluating worldbrains memex for firefox. It let's you full text search your visited sites the results can also be included in a google search via the plugin. Sites can be annotated, tagged.