Haven't tested but moving to resposive might trigger a user agent change.

Can the same be done on FF?

Firefox has a mode where you can choose from a dropdown of phone/tablet types, and it will set your user agent and screen size appropriately. I believe it's called "Responsive Design Mode."

That worked, thanks.

Dev tools > Responsive Design Mode in the top right, then back on the main page, selected a phone model. I had to reload the page for it to show the phone version of the site.