Comment 11 for bug 584758

Web browsers created in the United States, such as Netscape Navigator and Internet Explorer, use the letters U, I, and N, to specify the encryption strength in the user agent string. Until the US government allowed encryption with keys longer than 40 bits to be exported, in 1996, vendors shipped various browser versions with different encryption strengths. "U" stands for "USA" (for the version with 128-bit encryption), "I" stands for "International"--the browser has 40-bit encryption and can be used anywhere in the world--and "N" stands (de facto) for "None" (no encryption).[5] Following the lifting of export restrictions, most vendors supported 256-bit encryption.

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