Firefox has one feature the other browsers will never have - privacy.
A trivial example is the battery API. It's nowadays mainly used for tracking, but is an official webstandard. Mozilla decided to fuzzy it, so it lost its usefulness for tracking. Google didn't.
And I imagine, there's hundreds of similar examples at this scale, which as an average user you'll just never hear of.
One bigger feature is Firefox's Containers. It's based on work from the Tor Browser. Which also just illustrates what this partnership sometimes brings forth. Tor Browser is going to always be there, checking the Firefox code for privacy problems, and will suggest better ways of doing things, which Mozilla can just adopt.
For Chromium, there exist in principle similar efforts, like Brave, Iridium Browser, ungoogled-chromium, but these will always fight an uphill battle against Google and obviously Google isn't going to adopt and maintain their fixes.
Why should we trust them?
No US based entity can offer user privacy, as the government can force any of them to cooperate while requiring non-disclosure. The fact they don't use anonymous user data to offer internet services doesn't negate the fact that where most of us actually care about privacy, they don't have the power to actually offer it.
In any event, being a non-U.S. entity has less significance with each passing day with respect to the ability to insulate information from government. Consider that in the past decade, the U.S. managed to get the Swiss government to change its once-formidable bank secrecy laws.