We can't reproduce this. Tried on up-to-date Firefox and up-to-date MS Edge (a.k.a. Google Chrome).
Deleting a single key works.
Deleting multiple selected keys works.
Each case gives a prompt for which keys you want deleted.
Are you disabling JavaScript? If so, stop. Do you have page-modification extensions installed (like disable this JS function, or that JS function, or modify overlays, etc., etc.). Again, stop, and try again.
If it works without any extensions enabled, then begin re-enabling extensions until you can find the rotten egg and delete that one.