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.