Not very well, though.

NaCl supports JIT, basically enough for Mono. But it does not support self-modifying arbitrary asm code, which is absolutely necessary for fast JS, PICs need that.

Also, NaCl increases code size significantly due to its sandboxing mechanism.

Finally, NaCl performance is good on x86 but less so on x86_64 and ARM, again due to how it sandboxes (on x86 there are some nice tricks that make it work better).

Finally, you would need to port to NaCl. It isn't trivial, you need to use the Pepper APIs.

So you could run Firefox, but it would need to run JS in the interpreter, it's code size would be very large and performance would be significantly slower than a true native app. It would not be a competitive browser on a Chromebook due to those large limitations. So practically speaking, Chromebooks do not allow other browsers.