Ask HN: Is there a Firefox extension for persisting a viewport, even fullscreen

There are hundreds of ways to uniquely identify a user visiting a website.

I'm focusing on the screen/device type area: Screen dimensions, refresh rate, dpi, and user agent.

Firefox and Chrome both support a feature in their dev tools, where you can adopt the screen dimensions of a common phone or tablet model.

Is there a Firefox extension that allows this "viewport" to be enabled all the time? I'd love to know that no matter what dimension I set my Firefox window to, or when fullscreen, it's always rendering a 1920x1080 "generic" viewport within it. All it needs to do is make it comfortable for me to pan around/zoom in/out.

I haven't seen an extension quite like this.

(Thank you)