Do you accidentally click on the close button of Firefox's tabs? You can move them or remove them, by typing 'about:config' in the Omnibox and editing the preference for 'browser.tabs.closeButtons'. Here are the meanings of each value:
0: Display a close button on the active tab only 1: Display close buttons on all tabs (default setting) 2: Don't display any close buttons 3: Display a single close button at the end of the tab bar
