Update on this one: While responding to them regarding what version of FF this happens on, I went to check what version I was on (and having an issue with), and it prompted an update to Firefox. I went from version 71 (where I was having the issue, as shown above) to 72. Once upgraded to FF 72, the problem stopped happening for me.
So I believe it's simply a matter of... update your FireFox to the latest version if you're having this issue!