It totally depends on how the web site owner has setup his website. Lets say you are a site owner and when you detect a mobile web browser you redirect them to page a and when its PC user you redirect them to page b. Now by no means when you are at your PC you can view the page a because it will redirect you to page b and vice versa.
Now I think there should be a way to fool the coding out there but for that you should be doing some RnD.
First have a website page where you can get the details about your mobile page like browser type / OS. Now using the Firefox browser agent have your details as if you are on mobile and try browsing the website using that agent and you may have some success but I am not sure about it.