I think what they might be missing is that Mozilla's conclusion is a lot like the people seem to be thinking here -- even though Chrome doesn't have the fancy extensions that are possible in Firefox, many many more people are using Chrome than Firefox, because they see it as a better Firefox.
Clearly, that has something to do with either security or performance, where Firefox has a perceived lag.
Mozilla is actively working to improve performance, and you may not want to switch to Chrome after all, because Chrome may instead be the poor man's Firefox (I personally think this is already the case, simply because with my 679 open tabs, Chrome would simply kill itself [44 loaded]).
This video is one of the most exciting I have seen with regards to web browser performance, and it's Mozilla:
https://www.youtube.com/watch?v=an5abNFba4Q
More info on what this means for Firefox:
https://wiki.mozilla.org/Quantum
TL;DR: What if even with the missing XUL extensions, you wouldn't want to switch to Chrome, because Firefox was just plain faster?