Standardising firefox plugin installation

[David Nielsen]

It's come to my attention through painful experience that if you install
a plugin like gnash or the totem plugin there doesn't seem to be a set
standard for where to install these. This leads to many issues like
other gecko based browsers, like my beloved Epiphany, doesn't pick these
up.

The situation for gnash is even worse , it's not even picked up by
firefox (using about:plugins to display registered plugins), it is
however in the same directory as the totem-plugin so I think this might
be a gnash bug. 

We should definitely seek to get this resolved, for the benefit of users
as well as packagers of 3rd party plugins in Extras.

- David Nielsen