Unfortunately, it appears to be an issue with the way that Gmail is designed. Basically, the Gmail website displays in fullscreen, but each email that you open appears in a scrollable window within the big Gmail page. So when you are taking a fullscreen screenshot in Firefox, it's taking the fullscreen, but it just can't scroll that small window with the email in it. I hope I explained that clear enough. Unfortunately, there's not much that we can do to help you here. The best option is likely to [https://bugzilla.mozilla.org/ file a bug] on the Bugzilla platform and a Firefox developer will diagnose the cause and hopefully patch the issue in a later version of Firefox.