You need pkgconfig installed to compile this, even if using the
--enable-gtk1 option.

This plugin requires the 1.6 version of mozilla or gecko-sdk in order
to work correctly with the Opera Browser. The easy and smaller way is
to use the gecko-sdk-1.6. It doesn't need to be compiled. Just unpack
it and give the path in the configure options. You can also give the 
opera directory as the mozilla-five-home so that the plugins get 
installed to the opera directory instead of to your firefox, mozilla
or seamonkey path.

This plugin seems to work fine for use with any firefox, mozilla or 
seamonkey browser version.