In the latest Aurora and Nightly builds (12.0 and 13.0a1) Mozilla has introduced a new tab page like Google Chrome. As can be expected, this conflicts with Fast Dial's option to "Open in every new tab". Here are the step-by-step instructions on how to have Fast Dial to open in every new tab instead of Mozilla's New tab page:

UPDATE: The following fix is not needed any more. Install Fast Dial version 4.2.1 to have all these settings automatically change. Fast Dial 4.2.1

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That's it.