When typing in the search bar, there are a bunch of prefixes you can use to narrow what Firefox is filtering on. Type one of these characters and then a space to get more precise auto-complete suggestions:

• ^: search your history
• %: search open tabs (includes mobile tabs!)
• *: search your bookmarks
• +: search pages you've tagged (you can do this when you bookmark something, i never have but it's aspirational #aspirationalcomputing)