NoOb in proxy!! Need some Information!!

Hello!!
To be frank, I am a noob in this proxy thing and want to get some info regarding proxy switching.
I am a fan of chatting and went to a mobile site for chatting. In the previous version of the site I could see shortform of the countries in front of every user in the chat. So just by chance I just used the proxy option in my mobile browser setting tab and got my location changed to USA. But when I applied this trick to a PC site using the same option in Mozilla Firefox but didn't get the expected result.
So, the point is, can anybody tell what the PC site has got under its hood or in its scripting that it can filter the proxy I used in Mozilla Firefox setting and showed my original location? Thanks.

Your proxy was transparent/Level3 I guess which was revealing your ip.

Hi,

You either did not configure the firefox proxy settings correctly or the proxy you were using was transparent.

Was this a public proxy or private proxy?