It is quite simple but for firefox funs it will be useful!
1) Make a backup (anywhere handy) of the file /usr/share/applications/firefox.desktop
2) Extract the attached archive and replace the file /usr/share/applications/firefox.desktop with the generated file (named firefox.desktop as well)
3) 'Repin' firefox(uncheck "Keep in launcher", launch firefox, check "Keep in launcher") OR relogin OR restart unity (unity --replace)
4) Enjoy the new firefox feature you added by right clicking on the launcher's firefox icon:
Note that you can also add your own features you would like to by editing the .desktop file and seeing the firefox's available arguments by giving 'man firefox' in a terminal, as the whole thing is quite simple