When the URL prefix for mobile is set to 'm' and your domain name also starts with 'm', then you will be forced to use the mobile theme, regardless of your personal settings.
Webmin 1.5.6 on Ubuntu Server 10.04.3 LTS
Client: Firefox 6 on Windows 7 32 bit
Normally it is set to 'm.' rather than just 'm' . Do you have an actual domain that starts with 'm.' ?