That website is using an old method of checking the encryption level a web browser is capable of, it mentions Netscape 6.x/7 which are like from 6 to 7 years ago. It looks like they're checking for a U in the User Agent string. That particular letter in the UA goes back to the days of export controls for encryption software, but those controls expired over 15 years ago. That letter has been virtually meaningless since then.
http://en.wikipedia.org/wiki/User_Agent

You can fool that website into thinking that you are using an older version of Firefox with this extension.
https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/
Use this for a custom UA string that should work there:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.18) Gecko/20110614 Firefox/3.6.18

Or just use IE for that one website.

It is only a warning.
You can ignore that warning and click the Continue button.
That should work without problems.

Warning - We did not detect that your web browser supports 128-bit high encryption. If you have trouble connecting after you click the "Continue" button below, you will need to upgrade your web browser to a version that supports 128-bit encryption.

I would suggest you contact them about their incorrect browser sniffing issues as this change of U being removed in useragent has existed since March with Firefox 4.0.

Firefox easily supports 256-bit encryption even, if the website provides the support for it.

   https://www.fortify.net/sslcheck.html
   https://www.fortify.net/cgi/ssl_2.pl