I think you are right. I think MW code would have to be changed to handle keydown instead instead of keypress.

Same problem with Firefox 65.
I run most websites in Chrome but have specifically clung to Firefox for the MW formatting.

To confirm, i rolled back to Firefox 64 and no longer have the problem.

I'll need to do some testing and figure out what to do, here - it could be as simple as replacing all the keypress with keydown, or there could be rewiring of all the vbulletin and yui editor code