By default, Firefox 3 does warn for both closing a window with multiple tabs, and for quitting (regardless of the number of tabs/windows).
The Quit warning is disabled if you select "show my windows and tabs from last time" under "When Firefox starts:" in the "General" tab of preferences.
In addition, some extensions appear to set the "browser.warnOnQuit" pref to false, which completely disables the Quit dialog.
By default, Firefox 3 does warn for both closing a window with multiple tabs, and for quitting (regardless of the number of tabs/windows).
The Quit warning is disabled if you select "show my windows and tabs from last time" under "When Firefox starts:" in the "General" tab of preferences.
In addition, some extensions appear to set the "browser. warnOnQuit" pref to false, which completely disables the Quit dialog.