I think I've worked around this. It seems to be related to FF saving form values across refreshes, but it appears that it stores a value that should be the default value but differs with an extra <br>. I've added a handler to recognize that and it seems to work correctly now.
The only way I could reproduce it was to type, ctrl+a to delete the text, soft refresh the page and then click "reply" without focusing the editor (the issue went away when focusing). It has to do with the position of the cursor in that case.
Very impressive, Mike. I can't even imagine how difficult it would be to create a WYSIWYG editor in XenForo that works with all the major browsers and operating systems. My hats off to you, keep up the great work.