Besides, if you would like take a tiny amount of market share from Google (thereby weakening their ability to set standards such as the W3C's "EME" DRM, which they support), you know that using Firefox is the right thing to do.
It also shows Google that you're not buying into their free/nonfree paradigm that discourages developers from making apps/extensions not approved by Google's stores.
Also, Chromium OS, as distributed by Google, is not free of surveillance mechanisms such as the omnibar. You'd first have to audit, hunt and remove these features, or trust a third party to do this job for you.
Besides, in my experience, there are fewer good FOSS extensions in Chromium, in particular anti-surveillance ones, but that depends on what extensions you want.