It appears that there are now significant numbers of sites - or at least noticeable parts or features of sites - that rely on Google-specific APIs and haven't been tested on other browsers.

However visiting those sites from Apple devices is often similarly frustrating. I'm not sure this is an anti-Firefox thing. It seems more of a not realising there are other browsers apart from Chromium-based ones thing.

I have used Firefox exclusively every day on macOS for the past year, several hours a day, and have not one single time had to open another browser. None of the customers of my B2B saas use Firefox. Not once have I needed to test in their browsers except quick smoke testing to prove to myself that the rendering and details are identical. All my other browsing is indistinguishable from before when I used Chrome, and from on mobile where I (tragically) am forced by Apple's monopoly to use Safari.

Do you have evidence to support your false claim?

In case people don't know, Firefox was the first major browser to even have extensions.

I install uBlock Origin on every browser I come across. Everybody notices. The internet just feels better, somehow. People can't quite explain what changed, but they know.