Sometimes the simplest solutions are the best.  The FoxClocks add-on for Firefox allows you keep an eye on the time around the world – or just your local time – by putting small clocks in your statusbar.
This has been one of Mozilla’s favorite featured extensions for some time, due to the fact that well… people like to know what time it is?
Here is a little more about FoxClocks from the developer:
You can set each clock’s colour, or even have [...]