Mozilla has cachet with nerds like us because of its history. Not because it's particularly better than alternatives or because it has enough of a user base to throw its weight around.
In contrast, they do test on Safari. They'd lose double-digit percentages of users if they didn't.
The ecosystem, unfortunately, generally has room for three top browsers. I'm not even sure Firefox is number four these days.
For obvious reasons, I don't think Google is devoting any resources to optimizing the experience of navigating to google.com for people who don't want any history generated on their use of google.com.
I agree with you that the other features listed are positives, but they don't appear to be positive enough to push use of Firefox psst Safari, Chrome, or Edge numbers.
They didn't test on it at all. It's a browser with sub-5% market share.
There are folks in Google who personally test on firefox and believe in it ideologically as an important peace of the web ecosystem, but that's not a company policy and detecting Firefox bugs does not gate feature release for most projects.