Not sure if it is firefox, or what but the lock button doesn’t work. It just opens the customize items panel:
Snapshot: a4a45b136f5541e29b0a2445536b1618
The intent was to remove the gem from the belt and lock it.
We do not have a way to lock a socket empty. If you pick a gem from the list it will lock it in place, and the lock icon will turn orange to indicate that one or more of your gems are locked.