I was getting the same error message in Firefox today as well. ...just started using this plug-in today, I found a solution. Not sure if this is ideal/correct or not but the error message has gone away in Firefox and it's rendering the mobile version correctly on my phone.
There are four plug-ins in the Plug-in Manager. They are...
Mobile - Simple
Mobile - TeraWURFL
Mobile - Forever
Mobile - Domains
I turned them all off except for Mobile - Simple. It's working the way I want it to. Again, this might not be the solution you'd prefer because the other plug-ins are now deactivated.
EDIT: I just turned them all on, and left Mobile - TeraWURFL off. Firefox is not showing the error, and it's continuing to display the mobile template correctly on my phone.
The moment I activate Mobile - TeraWURFL, Firefox displays the error.