Pressing the numpad 5 key with NumLock off is supposed to generate the symbol KP_Begin. This works fine in Chrome, but not in Firefox. Tested with Firefox 70 on Fedora 30.
Apparently Firefox just gives us "Unidentified", so we can't really do anything clean here. Reported upstream for now: https://bugzilla.mozilla.org/show_bug.cgi?id=1592948