I don't think this is apples to apples, because ChromeOS tries as best it can to make the browser the entire OS, with any remaining Linux components abstracted

I don't think this is apples to apples, because ChromeOS tries as best it can to make the browser the entire OS, with any remaining Linux components abstracted away. So I wouldn't necessarily consider it anti-competitive that a Chromebook can only run Chrome, anymore than I would it consider anti-competitive that a Nintendo Switch can only run Nintendo Switch games. But to strictly answer your question, the Chromebook does allow you to turn on developer mode, where you could then install a Debian/Ubuntu chroot via crouton, on which you could then install Firefox. So in a word, yes.