RoundCube version 0.9.1
After login, right on the first page with the message list, current stable Firefox shows the message "This PDF document may not be displayed correctly" at the top of the page content. It disappears as soon as one changes to another page in that window/tab. I've observed this on several computers, and I believe I've already seen it in the past but never got around to report it.
You should update to newer version. It has been fixed.