Spoof Timezone от Joue

This extension spoofs the current timezone provided by JavaScript to web pages. You can specify the preferred timezone in the options page and the add-on makes sure your real timezone is protected (invisible by websites). There is an option to ask the extension to provide random timezones as well. This option is useful to prevent fingerprinting scripts from using your current timezone as an element to uniquely identify your browser.

Notes:
1. You can enable/disable the extension using the provided toolbar button.
2. To test the visible timezone use the right-click context menu item