I'm basically a "native" Linux user. When my parents finally decided to get a computer in 2008 or so (I was in elementary school back then), it got Ubuntu installed on it, so my first contact points with modern technology were 100% on Linux as anything invented after the 1950s wasn't used at all in elementary back then.
When I got my first own computer a few years later, the guy who guided my dad and me through building it suggested installing both Linux Mint and Windows on it. The Linux installation died on me after a few months for unknown reasons, I had no idea how to fix it and our helper disappeared into severe personal problems, so I used Windows only for quite a while until I finally started to really get into Linux inside VMs and was finally able to reinstall it on the bare metal.
As I had always prefered Linux, it quickly became my daily driver again.
Fast forward to today, Linux is the only OS on my laptop and the main OS of my gaming PC. I use Manjaro KDE on both.
I'm honestly astonished that Google hasn't pulled the plug on Mozilla yet. After all, their missions completely and utterly oppose each other and Mozilla probably causes the biggest losses to Google.
If your prediction comes true, which isn't unlikely, Firefox forks that already exist would probably take its spot. Or privacy friendly Chromium based browsers. I know, the latter sounds like an oxymoron, but they exist and one of them I would be hated on for naming has actually been proven to have better out of the box privacy than Firefox.