Can reproduce on your own site only, on latest Firefox, on both Windows and Mac.

Left-click doesn't work once text is there, right-click does work.

You had broken it with your "user-select: none". For everyone's stability I have put an override on CkEditor to undo that, as I see other people setting that too.

That's odd. user-select: none doesn't break it in Edge nor Chrome. But thank you.