Firefox sends your uptime

[Gregory Maxwell]

On Sat, Apr 5, 2008 at 6:01 AM, .FUF <fuf at itdefence.ru> wrote:
> Mozilla Firefox sends your computer's uptime while establishing TLS
>  (SSL) connection. This could be used to correlate anonymous traffic with
>  non-anonymous (e.g. LAN traffic) by correlating intercepted uptime
>  values (or to search the originator of anonymous traffic by correlating
>  uptime values from TCP timestamps in GNU/Linux and some other operating
>  systems)

Why does TLS include this?  Is it used for some kind of replay prevention?