I do the following
#cd /usr/ports/www/linux-firefox && make install clean
#cd /usr/ports/www/linux-flashplugin7 && make install clean
Flash plugin 9 will work, but it crashes a lot so I opt for flash plugin 7. In case the symlinks are not automatically created for you, all you need to do is
ln -s /usr/local/lib/npapi/linux-flashplugin/* /usr/local/lib/linux-firefox/plugins/.