A member pointed out to me that if he copies text already in the reply or new thread form then tries to paste the text back into the form it's invisible. Submitting the reply or clicking preview shows the text normally. I've confirmed this but it does not happen on this board and it only happens in Firefox.
As a troubleshooting step I globally disabled all plugins via config.php and hard refreshed the page. The issue persists even with the default style. I also did the file health check and it came back clean. Any ideas?