If you are using Mozilla Firefox 4.0 or later, you might have noticed a new feature. If you have only a single tab opened in Firefox window and you close the tab, the main window also closes and you need to re-launch the browser.
Actually its not a bug or problem. Almost all other browsers do the same thing and Mozilla has included this functionality in Firefox 4.0 and later versions.
If you don't like this new change and want to keep the main window opened even if you close the last tab, following method will help you:
1. Open Firefox and type about:config in the addressbar and press Enter. It'll confirm, click on I'll be careful, I promise! button.
2. Now type lasttab in the Filter box, it'll show following boolean key:
3. The entry "browser.tabs.closeWindowWithLastTab" would be set to true. Double-click on it or right-click on it and select toggle. It'll set its value to false.
4. Now whenever you close the last tab, Firefox main window will not close.