Firefox settings for this are in Tools -> Options -> Advance -> Network -> Settings Once, you in here, hit the radio button that states "Manual Proxy Configuration" The HTTP Proxy should be filled in with information that looks like this: 22.214.171.124. The Port should be something like 443, 8080, 80, 3124, or really anything. Proxies can be set to use many different/ all port.
EDIT: Here is another way of making it think you're from somewhere else without using another proxy. Go to Tools -> Options -> Content -> (In the languages section) Choose -> Select Languages to Add. Select the language you want and click Add. Now move that language to the top by have the language you want selected and click the Move Up button. Once your language is at the top click OK on both of the windows and you are done.
I hope this helps. If you need anymore help, let me know with a comment.