I used to work at Mozilla.

Safari isn't Chromium (it's the opposite, Google forked WebKit and they've diverged). But that's not really your point.

There's a lot of reason to use Chrome: deep integration with Google (privacy issues aside, it's really useful), better add-on dev ecosystem which leads to better add-ons, WebKit was far ahead of Gecko for a while, I personally prefer the devtools in Chrome, developers tend to verify their website works more in Chrome so fewer bugs, iOS is webkit-only, etc.

Firefox is a great browser, especially now. But so is Chrome.