I couldn't find a way to open the php manual in an already-open window except by compiling Firefox myself. Instead I had to go with re-launching the browser each time I wanted to check the manual. Also, in order to launch Firefox when it is already running, the new instance will need a separate profile, so it does get a litte complicated: In recent Firefox versions (2014 and probably earlier) there is a -no-remote option. On my GNOME system I use "gnome-open "; to open the default browser (Firefox). I've configured Firefox to open new windows in new tabs if a browser is already open.