It's a Firefox issue only.
At least I know how to fix it -- but not why it is a continuing issue with this site only. (If it were a Browser issue, you'd expect it to be a problem with other sites that seem to be using the same software. It isn't.)
(I should have hovered over the arrow to see what it's called. I haven't figured out a practical use for the REDO key, yet. <grin>)