If for some reason you need to reset defaults in Fast Dial and you cannot access Preferences please try the following:

Type about:config in the address bar
Hit go
You may see this screen

Click on I'll be careful I promise.

When you are in about config scroll to the following line, right click on the line and select reset:

That should reset your Fast Dial settings to default.
That should effect the following settings

These are the default values

{ width: 3, height: 3, thumbWidth: 300, timeout: 1000 }

It may be found useful to use about:config to reset other settings in fast dial to the default value.