One of the changes in Firefox 4: '''''by default''''' Firefox 4 does not display the "Save Tabs and Exit" warning message when closing multiple tabs, but it can be turned back on by changing some preferences. *<u>Saving tabs was not removed from Firefox 4</u>, but it works a bit differently. **By default, Firefox 4 will not ask to save tabs, but instead it will always save your tabs on exit. You can restore them next time by ***choosing "Restore Previous Session" from the History menu, or ***pressing the restore button on Firefox's default about:home home page. **''<u>Note: This was a decision by the developers. On this forum, we are all '''uncompensated volunteers''' here to help other users. The developes/employees of Mozilla/Firefox rarely, if ever visit here.</u>'' '''Browsing history must be saved''' *Firefox button: Firefox button > Options > Options > Privacy > [X] Remember my browsing history *Menu Bar: Tools > Options > Privacy > [X] Remember my browsing history '''Also make sure you do not clear "Browsing History" when using Clear Recent History or when closing Firefox''' *Using Clear Recent History **Firefox button: Firefox button > History >Clear Recent History **Menu Bar: Tools > Clear Recent History *When closing Firefox **Firefox button: Firefox button > Options > Options > Privacy (checked) > Settings **Menu Bar: Tools > Options > Privacy (checked) > Settings *See: https://support.mozilla.com/en-US/kb/Clear%20Recent%20History To display the "Save Tabs and Exit" message when closing multiple tabs, you can do the following: #Type '''''about:config''''' in the URL/Location/Address bar and press the Enter key #Accept the warning message (promise to be careful) #The preferences page/list will open #Filter = '''''browser.tabs.warnOnClose''''' #If its value is set to false, double-click on it (in the lower panel) to toggle its value to true #Repeat steps 4 and 5 for these following 3 preferences also: #*'''''browser.warnOnQuit''''' #*'''''browser.warnOnRestart''''' #*'''''browser.showQuitWarning''''' #**Note: last pref is new in Firefox 4b12, may need to add (boolean) #**Info/Instruction for using '''''about:config''''': http://kb.mozillazine.org/About:config #See: #*https://support.mozilla.com/en-US/questions/785169 #*http://forums.mozillazine.org/viewtopic.php?f=23&t=2105039&p=10434467&sid=fd2a35945bb2c633687f7b50d8c53989 #*http://blog.zpao.com/post/3174360617/about-that-quit-dialog

I'm sure you are using Private Browsing mode in Firefox 4. I have also the same problem and I tried to uncheck this option after many times accessing about:config\browser.warnOnquit. And it's wonderful, the warning cameback. I think it's a bug about Private Browsing mode. Hope Mozilla's Developers will fix it.