Edited again: that's great, thanks! Any chance we can get more than one star on your review?

We can reproduce this, both on and off Instagram. It seems to be associated with Firefox 66.

Edited: fix is up, please look for 4.0.76. Thanks again for the detailed report!

Working on a fix now but it's going to be a couple of days at least. We have had some luck reloading the page or pressing Esc to close the overlay and pressing the browser button or hovering Save button again, but it's inconsistent.

Thanks so much for the detailed report!