In later Firefox (Quantum, now testing with v.70 in Linux), it's
* (instead of
+ of the initial answer).
* can be typed before or after the search terms.
So, in order to search
Tags: work,book, type:
* is too difficult, an addon that works at the date of this post provides a button that triggers a field where the key words can be typed: Search Bookmarks Button.
Some application launchers can search among internet bookmarks. In Linux there are many solutions, I hope there are in other systems too.
Gnome and Plasma desktops have search tools with that capability already.
When lacking that feature in Linux, one could use Synapse as it is desktop-agnostic. Synapse has a plugin to search Chromium bookmarks, but not one for Firefox; having Chromium installed and importing Firefox bookmarks in Chromium does the trick.
So, enabling the plugin:
search your terms and press the right-arrow key to select the "Internet" category to the right: