(Firefox) Pop Ups don't display after switching virtual desktop

I got a very strange issue when switching from virtual desktop to another:

Firefox Pop Ups are not shown anymore when i switch to another virtual desktop an immediately switch back.

To get a common base to talk about:
By saying "Firefox Popups" i think of such things as

* the notification (displayed as a yellow bar beneath the address entry field) of the popup blocker. The notification itself is displayed, but not the properties window where you can choose to block the pop up or to show it.
* the right click menu. It is shown, but as soon as you move the mouse onto the popup, it's gone.
* any popups/drop downs displaying suggestions when you're entering text. Eg. the address entry, or entries on websites

This strange behaviour occurs when:

I switch from the desktop with the firefox window to another one and then immediately back, keeping STRG and ALT pressed.
If i then switch to another desktop and the release all keys, and the switch back to the firefox window, everything works as excpeted again.

Please tell me what extra information you need, as I don't know exactly where the config of unity/compiz is stored.

I'm on a lenovo T500 with intel graphics using a complete up to date and default natty 64 bit install.
The issue is connected to compiz/unity, because everything's fine when running gnome or unity2d