Sadly, they aren't really living up to that point.
Firefox OS works basically like AOSP - it's up to vendor to provide you unlocked environment (or not). It's not even on a requirement list when you want to market your device using Firefox OS brand. Phones like ZTE Open often came with blocked bootloader you had to "jailbreak" to upgrade FxOS by yourself - seems familiar, doesn't it?
Some time ago I tried to compile Firefox OS to GTA04 - a hardware upgrade for Openmoko Neo Freerunner. It's pretty much the most open mobile phone out there these days, allowing you to run basically any GNU/Linux distro you'd want on it. I failed - available documentation is lacking; or actually, it focuses on something completely different. It assumes that you either have a Android phone and goes to great extent describing how to extract non-free binary blobs from your current system and apply them to FxOS, or you have a great knowledge of Android build system and you can handle everything by yourself. GTA04 isn't Android based - while there is Replicant build working on it, it's still closer to standard PC way than Android phones when it comes to stuff like booting etc. Not having much experience with Android from developer side, I simply got lost in all those clearly Android-based instructions.
You'd at least think that Gonk, the hardware abstraction layer of Firefox OS implemented on top of Android base system, would be just a compatibility measure for dealing with vendors that only provide Android support. It's not the case - even though Honk (a Gonk reimplementation based on standard GNU/Unix way of how systems work) exists, it was contributed as a part of experimental Raspberry Pi port and is not really maintained ever since. It makes me sad, cause Firefox OS had the potential to actually move smartphones in general towards being a little more open platforms, which is always close to my heart as a long-time Openmoko and OpenPhoenux supporter and contributor, but sadly this aspect got basically ignored by Mozilla itself and they went with pragmatic "let's just make it work, even if it means having non-free low-level androidware under our free OS layer" :(