What are all possible ways a keyboard shortcut could be bound in Safari?
I've run across this post on StackOverflow that addresses this.  Basically, there's no central repository of all the shortcuts created because, in the end, the application isn't required to register the event with the system.  

However, they did recommend an application (KeyCue from Ergonis, €19; Free Trial) that has the ability to find all of the shortcuts that are assigned to an application (i.e. Firefox).  I downloaded and tested the application and it seems promising.

(Thanks to user Nimesh Neema - see comments below), you can also use CheatSheet ($0; free as in beer).  Both work in the same manner with roughly the same output (KeyCue is shown below).

Holding down the ⌘ Command key for a couple of seconds brings up an overlay showing all of the keyboard shortcuts that are assigned to the "front most" App - in this case, Firefox.



If you're searching for a particular shortcut you'll have to go through each running app manually to find it.  So, as stated in the SO answer, while not perfect, this will get you close to what you're looking for.