Or are you talking about viewing different times around the world in
general? If in general, there is such a plugin. It is called FoxClocks.
See what time it is locally, in NY, LA, London, and other cities. You
can choose the cities and the time display format. FoxClocks is
available from Mozilla Update, and it works with both Firefox and
Thunderbird. The description there says it also integrates with Google
Earth.