Open external links in new windows or tabs
If another program wants to display a web page in the default browser, Firefox will reuse an existing window by default, which means that it will navigate from a page you might be reading. To stop this, Go to Tools > Options... (Edit > Preferences... under Linux and Mac OS X), select Advanced and click on Tabbed Browsing. Under “Open links from other applications in:", you can choose the alternative you prefer.