browse-url-firefox-new-window-is-tabbrowse-url-firefox-new-window-is-tab is a variable defined in `
browse-url.el'. Its value is
nilDocumentation: Whether to open up new windows in a tab or a new window. If non-nil, then open the URL in a new tab rather than a new window if `browse-url-firefox' is asked to open it in a new window. This option is currently ignored on MS-Windows, since the necessary functionality is not available there. You can customize this variable.