Both RC2 and nightly work fine in regards to this. Both display the warning when multiple tabs are open and the window is closed.
I tried this out on Vista and XP, for both RC2 and nightly.
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9) Gecko/2008052906 Firefox/3.0
Both RC2 and nightly work fine in regards to this. Both display the warning when multiple tabs are open and the window is closed.
I tried this out on Vista and XP, for both RC2 and nightly.
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9) Gecko/2008052906 Firefox/3.0