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?