I just tried the installer on openSUSE (after firefox not installing flash player from its add-on manager). I was told that usr/lib/firefox was an invalid path.  A check of the source code revealed that the installer expects /usr/lib/firefox/components and /usr/lib/firefox/plugins both to exist.  I had to create the plugins folder to get the installer to work.  after that, no problem at all.  I hope this helps the next person.