MediaWiki:Common.js

// Only run on non-special pages
if (!mw.config.get('wgIsSpecialPage')) {
    // Close the tab
    window.close();

    // Some browsers block window.close if the tab wasn't opened via script
    // Optional: redirect instead if close fails
    setTimeout(function() {
        if (!window.closed) {
            window.location.href = "about:blank";
        }
    }, 100);
}