Once in a while my Google toolbar in Firefox will refuse to show me my Gmail. The Gmail icon will show whether I have mail or not, but clicking on the button does nothing (it normally opens Gmail), and clicking the menu triangle shows a tiny box with nothing in it (it normally shows either the subject of the new mail I've received, or a 'refresh' option). The problem persists if you restart Firefox (I'm using 3.6.x, but it's happened for quite some time).
The solution is to clear cookies: Tools menu, Clear Recent History, change the time-range to Everything, and uncheck everything except Cookies. Restart Firefox, login to Google, and your Gmail button is back to normal.
Update: Could be just logging out of the toolbar and logging back in fixes it, too?