With Undo Close Tab option, the closed tab will open in a new browser window. It should instead open in the same group view that it was just closed in Repro: 1) install nightly Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0b4pre) Gecko/20100815 Minefield/4.0b4pre 2) Create a bunch of tabs and launch them in a group view 3) close one of your tabs, then reopen it by context clicking the tab bar > Undo Close Tab 4) Verify the closed tab is now reopened in a new browser window Expected: - closed browser tab should be reopened in the same tab bar group it was just opened in Actual: - reopened in new browser window

I think this is a duplicate of bug 586198.

The tab wasn't opening in a new window. The other tabs were actually getting hidden, but if you tried to switch windows, you would see that there's just one window.