when you do "about:debug" it will appear that nothing happens. However, when you click the menu icon (top right corner of the screen), you see some more options.
You are suppose to be able then to click settings, and there will be a debug option. Click that, then select "UAString". From here choose "Desktop".
Back out of everything, then type "about:debug" in the address bar again.
I will say though, I have seen some sites that continue to dump me into their mobel sites, even though I have set this. But, then others I'm getting to the normal site. Not sure what that is about...