This is due to the issue reported here: http://forums.zotero.org/discussion/92/
In short, it's a Firefox bug that causes the wrong tooltip to appear on menu items. (Try hovering over Reload in the Firefox toolbar until "Reload current page" appears, then quickly click the arrow for the drop-down in the Forward button and hover over one of the menu items. You'll still get "Reload current page.") The empty tooltip is a workaround. There's surely a better workaround that could hide the tooltip completely, but unless someone contributes a patch, it'll probably stay this way until Firefox 3.0.
Upgrade Storage
In short, it's a Firefox bug that causes the wrong tooltip to appear on menu items. (Try hovering over Reload in the Firefox toolbar until "Reload current page" appears, then quickly click the arrow for the drop-down in the Forward button and hover over one of the menu items. You'll still get "Reload current page.") The empty tooltip is a workaround. There's surely a better workaround that could hide the tooltip completely, but unless someone contributes a patch, it'll probably stay this way until Firefox 3.0.