For me, it opens in the same window if the window has a tab bar .. if it doesn't (e.g. it was spawned from Javascript in another window specifying no menu bar, a status bar, etc) the tab will open back in the creator's window.

Firefox 3.6 used to create a tab bar (in the window containing the link) and then open a new tab in this same window.

When Firefox 4.0 reloaded my saved windows/tabs from 3.6 after the install I had a window (created without menu, status bars etc) in which I had created additional tabs. It showed up without a tab bar and with the first tab visible. When I closed the window it prompted me to ask if I wanted to close all 3 tabs.

It shows up in History/Recently Closed Windows as '<Tab-1-name> (and 2 other tabs) and can we 'reopened'. It still doesn't have a tab bar but Ctrl-tab and Ctrl-1, Ctrl-2, Ctrl-3 will show the other tabs.

It also seems to open another window, instead of a tab, if I only have one window open.