You don't need to have anything listed in Custom useragents - it is disabled by default and I suggest you leave it that way.
I'll remove the default "firefox" text which was in there as a kind of reality check - you can test things are working by enabling the Custom section and viewing your site with Firefox.
You could however use the "Custom/User Defined Useragent String" to display a specific style to Firefox users only (it doesn;t have to be mobile)
|