When using "request desktop site" in Orfox, a mobile phone User-Agent gets sent instead of a desktop browser User-Agent
What happens
When I request a web page normally, the following User-Agent header gets sent by Orfox:
Mozilla/5.0 (Android; Mobile; rv:52.0) Gecko/20100101 Firefox/52.0
This is the correct behavior.
However, when I select Menu > Request desktop site, the same User-Agent is also sent. (You can see this e.g. here.) Because of this, websites will still return the mobile version of a website instead of the desktop website.
You can try this at, for example, google.com.
What should happen instead
When using "Request desktop site", a User-Agent for a desktop browser should be sent instead of that of a mobile browser. The normal Firefox for Android does this correctly.