Stuck Maximised

[Jonathan Allen]

Dear Fedora List,

Going to maximise thunderbird on a graphic screen session, I accidentally
clicked on the box next door and it maximised to being absolutely full
screen, with no top bar and all the normal desktop stuff obscured.  I can
still switch tasks using Alt-Tab, in which case the top bars and the
desktop (I'm using Xfce) reappear for the other task(s).

How can I get thnderbird back to have a normal task appearance?

Jonathan