I don't use foxifi but I will guess it is the same no matter what app you use, its just the data you are using it with.
So here goes. (Now I might be wrong on the terms there, but its kind of the same I guess)
If you tether your phone data to your computer, your provider (t-mobile in this case) will know about it based on the packets your browser sends. You can bypass it kind of by installing an extension (on Chrome or FireFox) called
User-Agent Switcher. What it does it "makes" your browser appear to be android or iphone based, or any other client out there.
The downside with this is that pages that have a mobile page version to them, will start to appear in their mobile format. Most of them have the "full desktop" or Full website" feature on the bottom of the page usually.
T-mobile allow you to tether 500MB after that you get the message you need to upgrade your plan. So change the browser settings.
I am not sure about using it for other stuff. like gaming etc. Although, thanks to the fact that I now own the nexus 5, I get to enjoy a speeds faster then my home internet.
So enjoy.
Let me know if it worked for ya
