Can OSX proxy settings vary by browser? I'm moderately familiar with OSX but am attempting to solve an issue remotely.

My girlfriend is in Argentina for a bit, and the IP location bars her from many "US only" sites that she frequents.

To combat this, my proposal is to Install another browser (Firefox in this case) and rig that through a U.S-based Open Proxy server. This would be her "US" browser.

However, I wasn't sure whether changing the proxy for one browser's settings would change it in a central location for others. 

So, long question short: can I configure proxy settings for one browser only on OSX, or do they carry between all browsers?

Thanks in advance for any help you can give!
Any application can network directly or use the system settings. As @geekosaur states, Opera has its own setting, as does Firefox (along with the option to use the system settings). Chrome and Safar use the system settings.

It's really up to the application developer how they implement their proxy controls (if they even have proxying!).