Move or remove the close tab button
Do you accidentally click on the close button of Firefox’s tabs? You can move them or remove them, again through about:config. Edit the preference for “browser.tabs.closeButtons”. Here are the meanings of each value:
0: Display a close button on the active tab only 1: (Default) Display close buttons on all tabs 2: Don’t display any close buttons 3: Display a single close button at the end of the tab bar (Firefox 1.x behavior)