By default, Firefox works like other web browsers but it also provides an option to configure it differently. Type about: config into the address bar and press Enter to show all the configuration settings. Now, type middle into the search box. Doubleclick ‘middle mouse. paste’ to set it to True, and from now on, the middle mouse button will paste the contents of your clipboard. If you copy a URL, for example, middle-clicking in the address box pastes it in. Double-click ‘middle mouse. paste’ again to set it to False and return the middle-click to its normal function.