Firefox only (Windows/Mac/Linux): Firefox extension TabCloser adds a tab context menu item that quickly closes all tabs from web sites with the same domain name. Once installed, simply right-click on the tab and choose "Close all hostname.org tabs" to kill all similar tabs. The only irritation is the "Are you sure?" confirmation message that doesn't appear to have a setting to disable—though it might save you from accidentally closing something important. Readers using the previously mentioned Tab Mix Plus extension already have the equivalent functionality hidden deep within the settings. To enable, open the Tab Mix Plus Options panel, navigate to Menu -> Tab Context Menu and check the box for "Close Similar Tabs" to enable the menu item. This extension could be really useful the next time you get lost 20 tabs deep on Wikipedia doing "research" when you should be actually doing something productive. TabCloser is a free download, works wherever Firefox does.