Hopefully this doesn't duplicate something already posted that I can't find, but I can reliably reproduce this using Zotero 2.07b4 with Firefox 3.5.3 on Mac OS X 10.5.8.

1. Press CMD+Shift+Z to start Zotero. Zotero starts up, filling the bottom quarter or so of the browser--all as expected.

2. Press the "Toggle Fullscreen Mode" button. Zotero now fills the browser window, and the button appears pressed--still as expected.

3. Press CMD+Shift+Z to close Zotero. Zotero disappears, again as expected.

4. Press CMD+Shift+Z to start Zotero. Zotero appears at the bottom of the browser window, but its appearance is not as expected--it fills the bottom half of the browser window (perhaps slightly more than half), and the "Toggle Fullscreen Mode" button appears pressed.

5. Press the "Toggle Fullscreen Mode" button. I can't figure out if the behavior that follows counts as expected or not--Zotero shrinks down to the bottom quarter or so of the browser window, and the "Toggle Fullscreen Mode" button appears unpressed.

In any event, the problem is that I would expect Zotero to start up in (4) either covering the entire browser window with the "Toggle Fullscreen Mode" button pressed, or covering the bottom quarter with the button unpressed. I think I could handle either of those, but what actually happens after closing Zotero in fullscreen mode is a bit weird.