This question comes up now and again, and I don't know the answer. Indeed, I didn't know that Thunderbird has a full screen mode.

F11 is a common keystroke for toggling between full-screen and normal. But I feel that you have stumbled across a so-called "undocumented feature" and this full screen mode isn't meant to happen, and so may well not respond to the usual controls.

Are you using Windows or something else?

I'm using a Mac and the full-screen option appears top right, irritatingly it sits over the 'list all tabs' icon when in full screen. FYI the usual command to leave full screen (when browsing in Firefox for example) is the 'esc' key (top left) but it doesn't seem to work for Thunderbird! Sadly F11 doesn't work either – any other ideas?

I'm not familiar with OSX or Macintosh in general. It seems that full screen mode is built into the operating system.

Does this tell you anything you didn't know already?

Thanks Zenos but I'm afraid that doesn't help as I'm running Mac OS X Lion 10.7.5 not Yosemite. I did try what was recommended generally for Macs but it didn't work. Looks like I'm stuck with quitting out of the application – at least that works!