Tap the three button menu in the upper left and then check the desktop site box. Here's a screenshot for reference: http://imgur.com/gallery/9ozo9md

There are also many different extensions that can allow for more fine grained control should you need it.

Thanks, but that only changes it temporarily, as it reverts to mobile every time Firefox opens. I want to change the default setting to Desktop, so it does not switch back every time.