Yeah, ChromeOS is a linux distro that boots chrome and is now being officially split into Lacros (Linux and ChromeOS), but honestly, we should be able to make a “distro” that just starts a DE with firefox or chromium as the main window. Extensions could be written that present the filesystem in firefox or chromium and most users would be none the wiser.
Call it WebOS or something.
Does something like that already exist?
No but you can easily make it. Install Arch (which I use, by the way), set it up to launch a lightweight window manager and set that up to autostart Firefox in full screen mode.