World Clock is an add-on that lets you render world clocks in your browser toolbar popup.
Simply add as many clocks as you want from the drop-down menu in the popup. Please note that currently, 34 items are available to choose from. Before adding any item, you can set the daylight-saving value as well (i.e. -1, 0, and +1). Items in the toolbar popup are draggable, so you can drag them to your desired position; all changes are auto-saved to the memory. By default, 9 items are added to the toolbar popup, but you can remove them at any time by clicking on the cross-sign next to each item (remove button at the top-right corner).
If you have a feature request or found a bug to report, please fill the bug report form in the add-on's homepage.
