Firefox 4 and later versions save the previous session automatically, so there is no longer need for the dialog asking if you want to save the current session and that is what happens if you close the only opened window.

Use "File > Exit" or "Firefox > Exit" (Mac: "Firefox > Quit Firefox") if you want to restore multiple windows or otherwise have problems with restoring tabs.
You can use "History > Restore Previous Session" to get the previous session at any time.
You may need to click the orange/gray Firefox button to see History.
There is also a "Restore Previous Session" button on the default about:home Home page.

just why it happen if u open single window u can try it at ur end. I think it always ask during closing by close button considering accidentally hitting close button while you open multitab Second it is other thing to restore previous session

think again......

Can firefox fix this issue ........ i tell again . when you open first window of firefox and in it you open multitab and then if you click on close button firefox closes without asking/warning but if you have opened another window of firefox and in second window you try the same then firefox ask/warn that you are sure to close. It is a fault however firefox i am using and i found it pleasant to browse with firefox

You can set the warn prefs on the about:config page to true via the right-click context menu or toggle the value with a double left-click.

• browser.showQuitWarning, see http://blog.zpao.com/post/3174360617/about-that-quit-dialog
• browser.tabs.warnOnClose, see http://kb.mozillazine.org/About%3Aconfig_entries
• browser.warnOnQuit , see http://kb.mozillazine.org/browser.warnOnQuit
• browser.warnOnRestart , see http://kb.mozillazine.org/browser.warnOnRestart

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold (user set).
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean)

Thanx corel for reply . what you written i tried and set default but still the same problem can you check or guide me ?