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."
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.
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.