The address bar has become our entry point to the internet these days. Firefox in its default configuration does some sort of smart guess on what you type there. If it resembles a URL then the browser makes that request. If not, it sends the string you typed to your default search engine. It also includes some fuzzy search matches from your history and all that, which is fine 90% of the time, but sometimes you need a bit more control over what results it shows you.
This is a list of modifiers you can set at the beginning of the search to tell firefox what do you want to see on the results, a kind of filtering:
^ to search for matches in your browsing history.
* to search for matches in your bookmarks.
+ to search for matches in pages you've tagged.
% to search for matches in your currently open tabs.
# to search for matches in page titles.
$ to search for matches in web addresses (URLs).
? to search for matches in suggestions.
So, if you want to search for the word headphones in your bookmarks only,
you can type on the address bar:
*headphones
And, if you want to include only the results of your browsing history it would be:
^headphones