You can set the chromebook in developer mode and have a full linux system with all the software you want.

Or you can install the android version of firefox.

So yes, it is allowed.

It's a minor detail, but I feel there's a difference between "allowed" and "possible". Developer mode has a bunch of caveats that neuter many of the selling points of ChromeOS and I'd argue is more akin to a "jailbreak", just one that Google doesn't play cat and mouse trying to stop you from using.

The Android term "rooting" would be closer than the iOS term "jailbreak", precisely because of the reason you cited: Google doesn't lock down ChromeOS like a jail.

Good call. I agree that's a more apt comparison. That said, it's still really locked down, it's just there's a key on the wall next to the door with a sign saying "beware of the gremlins".

You can still install the android version though, it is actually the same core as the desktop version and supports extensions like adblock and noscript.

What you can not do is replace the "os shell" that is chrome without developer mode, but I'm not sure that would even work with firefox even if it was "allowed".

Firefox for Android can be installed on Chromebooks. So yes, other browsers can be installed on Chromebooks.