A single "U" (or "U;") in a user agent refers to the encryption strength that the browser supports.
The "U;" indicates that the browser supports 128 bit encryption. This was added in the 1990s when the US Government regulated the export of strong encryption from United States borders; not allowing encryption with keys longer than 40 bits.
Sometimes instead of the "U;" (which indicated "USA") there will be an "N;" - which indicates "No security".
Alternatively, there might be a "I;" - for "International" - which indicates weak (40 bit) encryption.
The export ban was lifted in 1996 and web browsers are capable of supporting much stronger encryption these days, so this flag is a bit of a relic of those times. It has been removed from Firefox since Version 4.0, and most browsers don't have it either.