OS X 10.6.6, '''''dual monitor config''''' on a Mac mini (3,1) from 2010. Just got FF4. Steps to show buglet (not checked on a single display Mac—you must try with above config, also running "Spaces"): If running FF 3 with “remember windows from last session”, close all windows, then quit FF 3, then relaunch FF3 to confirm it opens only the home page. Close that window and quit FF 3. Install FF4. Launch FF4. Close whatever welcome window appears. Open a new window with cmd-N. Repeat the previous step. As many times as desired. All the new windows are listed in the Window menu, but are not selectable in the menu. Similarly, pulling a new tab out to sprout a new window makes the tab vanish. Again, the Window menu reports the existence of the “pulled-off” window, but the window itself is not accessible.