What version of Firefox are you using? To find out which version, go to Help, scroll down and click on About Firefox.

If websites complain about 128 bit encryption not being available then that can be caused by sniffing the user agent for "U;" that was removed from the user agent in Firefox 4+ versions.

You can ignore this warning and see if it works if you continue.

Firefox supports 256 bit encryption ciphers (AES-256) since 2003 in Firebird 0.6, so all Firefox versions support 128 bit and stronger 256 bit encryption (128 bit shouldn't be used these days).

• https://www.fortify.net/sslcheck.html

• http://en.wikipedia.org/wiki/User_agent#Encryption_strength_notations
• Bug 572668 - Remove the crypto strength token from the UA string

Is there a particular website saying you need 128-bit encryption?

I agree with cor-el that it may be due to the site sniffing for the outdated U in useragents.