Is there a way of searching Firefox bookmarks by multiple tags without going into the bookmarks menu?
This feature is built into the address bar.
All you need to do is type a +, a Space, and then each tag, separated by a Space. So, if you wanted to search for all items tagged with both "comedy" and "video", you would type + comedy video directly into the address bar.
Source: Awesome Bar - Find your bookmarks, history and tabs when you type in the address bar