There's an open investigation about the Message on invoice or Message on Statement field showing a long stretched box in Google Chrome or Firefox. It still works as normal, it just has a scroll bar and only one line is visible at a time.
Our Engineering Team is currently investigating this situation and we're unable to provide a specific timeline on when this will be resolved.
I recommend contacting our QuickBooks Online Team so they can add your account to the investigation (INV-38382). This way, you'll be able to receive an update via email regarding the issue.