If Firefox adopts Chromium, there becomes effectively no way for Mozilla to shape the futures in manners that are not blessed by Google. For instance, Google doesn't seem to be interested in WebVR/WebAR, so there is no way to bring a Chromium-based browser in this direction until Google says so. The same thing happens with compression formats, for instance.
Also, as Google controls Chromium, a monopoly of Chromium also means that Google can decide to deprecate and/or customize web features without any counter-power. For instance, Google could decide to restrict WebPay to GooglePay, effectively leveraging their monopoly against PayPal and credit cards. There are also a number of manners in which Google could, in theory, make Netflix or Facebook or any competitor less usable or entirely unusable on the web. No Chromium-based browser can do anything against this.
Don't forget that Mozilla is also working on a next-gen rendering engine, Servo, that is expected to kick the ass of both Gecko and Blink on recent platforms, in terms of both performance, battery use and security. Making Chromium the only web browser means abandoning any hope to switch to this new architecture, as everything would need to be bug-for-bug compatible with Chromium, instead of following the standard.
(edit) typos