Firefox from www-client/mozilla-firefox-bin is 32-bit from the "manufacturer" Mozilla.com, and cannot use the now 64-bit version of net-www/netscape-flash 10.x.
Symptoms of it include inability to play Flash content (it just doesn't appear), and if run from the command line, Firefox will repeatedly mention that the plugin has the wrong class "[ELFCLASS64]".
Steps to Reproduce:
Workaround is ditch the binary and compile it yourself with www-client/mozilla-firefox so both Firefox and Flash Player are 64-bit.
I file this bug as a note, and will close it as an upstream problem.