You are clicking on the Close Window button. The methods to '''close tabs''' that I use in descending order are: * double-click on the tab, for which you need the '''[https://addons.mozilla.org/firefox/addon/tab-clicking-options/ Tab Clicking Options]''' extension. * Closing the tab with the close tab button placed at the right side of the tabs strip, see http://kb.mozillazine.org/Browser.tabs.closeButtons * Closing via the tab context menu, including additional options added by the [https://addons.mozilla.org/firefox/addon/multiple-tab-handler/ Multiple Tab Handler] extension (see my [http://dmcritchie.mvps.org/firefox/multiple_tab_handler.txtt notes]) More information: * [http://dmcritchie.mvps.org/firefox/firefox.htm#ext_table Firefox Customizations (Notes)] * [http://dmcritchie.mvps.org/firefox/tabs.htm Tabbed Browsing in Firefox] * [http://userstyles.org/styles/24728 Tab Color Underscoring active/read/unread (Fx3.6) - Themes and Skins for browser - userstyles.org] * [http://userstyles.org/styles/9043 Tabs Bar Minimal Size - Themes and Skins for browser - userstyles.org] If you are totally confused by 4.0, you can make Firefox 4.0.1 look like Firefox 3.6.17, see numbered items 1-10 in the following topic [http://dmcritchie.mvps.org/firefox/firefox-problems.htm#fx4interface Fix Firefox 4.0 toolbar user interface, problems (Make Firefox 4.0 look like 3.6)]