I could not find any way to do this the last time I checked (previous version, probably).

Some mobile pages have a "full site" link, usually toward the bottom.

To submit suggestions for new features, you can use this site:

https://fxos-ios-browser.ideas.aha.io/ideas/new

Or the Settings dialog:

• Tap the tabs button
• Tap the gear/wheel/cog icon
• Tap "Send Feedback" to open the feedback page

Request Desktop Site is coming in a future release. You shouldn't have long to wait!