The UserAgent sent by any browser tells the website that you are using a mobile device and the website sends the appropriate format for the device being used.
You need to change the UserAgent that is being sent
https://addons.mozilla.org/en-US/firefox/addon/59/

For mobile Firefox, try this add-on instead: https://addons.mozilla.org/en-US/mobile/addon/162014/

I have that installed but it still shows me the mobile version of the site. The desktop icon is grayed out.

After installing the "Phony" add-on, did you go to the mobile Firefox preferences and change the "User Agent" setting?

Try changing it to "Desktop Firefox" or "Internet Explorer."