Firefox 4 always stores the old session, and you can access it by going to the History menu and selecting "Restore Previous Session"

If you want Firefox to display the message to save the session, it can be turned back on by changing some preferences.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Locate the preference browser.tabs.warnOnClose, if its value is set to false, double-click on it to change its value to true
4. Repeat this for these 3 preferences browser.warnOnQuit, browser.warnOnRestart and browser.showQuitWarning

If you always open the last set of tabs, an alternative approach is this:

1. Click the orange Firefox button, then select options to open the options window
2. Go to the General panel
3. Change the setting "When Firefox starts" to "Show my windows and tabs from last time"

thanks TonyE, I do this steps ... it works properly !!

TonyE this is not working for me.I made all steps but when i close my firefox does not ask me.Any other suggestions ? Ty

My preference browser.tabs.warnOnClose is already set to True but all the other preferences don't exist in my version. I'm loathe to reinstall as I just finished setting up Zotero and don't want to have to do that all over again.

Is there any way to write these preferences in?