User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:2.0b10pre) Gecko/20110122 Firefox/4.0b10pre Build Identifier: Mozilla/5.0 (Windows NT 6.1; rv:2.0b10pre) Gecko/20110122 Firefox/4.0b10pre after i updated 22/1/2011 build minefield, i found that the warning of closing multitab is malfunction even i checked back the preference and use the safe mode and reset the preference . Reproducible: Always Steps to Reproduce: 1.open 2 or more tabs 2.press the x close button at the top right 3. Actual Results: it close without warning

This is expected behaviour from bug 592822, if you want to restore your session you can make that decision at startup from the History, Restore Previous Session option.