what is the encryption rate/cipher strength for Firefox 3.6.13?

Firefox can use AES-256 bit if web servers support it (this site does).
See How do I tell if my connection to a website is secure?

See also https://www.fortify.net/sslcheck.html

Firefox supports TLS1.0/SSL 3.0 using (at least):

3DES (56*3bit)(Paypal) RC4 (128bit)(Google , and etc) AES128/256(most commonly used) Camellia (256bit)(Geotrust, and etc)

All of them are literally safe, with the Electronic certificate system ,RSA/DH cipher key exchange methods, and SHA1 + MD5 cheksum hash algorithm.

Since SHA1 and MD5 are not strong enough, the higher version of TLS(1.1/1.2) use SHA512 instead.

See "Transport Layer Security" on wikipedia for detailed information

http://en.wikipedia.org/wiki/Transport_Layer_Security