iDevices are locked down for the purposes of Apple maintaining iron-fisted control over the platform at the expense of both developers and end-users. Chromebooks are locked down for end-user security and ease-of-administration.
Chromebooks allow the end-user to unlock the bootloader and/or run them in developer mode if they really want to do that (and this is all well documented by Google, not akin to jailbreaking), and pretty much the only reason a lot of developers even consider Chromebooks as full laptop replacements is for this reason.
Want to run Firefox on a Chromebook? Install crouton (a project developed by a Google employee) and just go ahead and run Firefox, works fine, just like any other Linux app. Google does nothing to stop this (in fact, they go out of their way to make tools to enable it), they just put enough friction into it to let you know that when you do this all bets are off as to the security of the chroot you are running native Linux apps in.