How to disable sound when switching Firefox tabs with ⌘1-⌘9
That’s certainly not a Firefox sound, at least not one that I’ve heard in the past. 


Do you have any Extension? Try disabling them all for the sake of testing.
Any other application running that could be using the same shortcut? (Safari uses that to access the quick bookmarks where ⌘1 corresponds to the 1st on the left (next to Top Sites) and so on. However, the lack of bookmark, doesn’t produce a sound on my system. I even have “Play User Interface sound effect” checked in Sound Preferences pane. 


My bet is that you have something else running and that is causing the “bonk”.