The majority of Mozilla and Apple's revenue for their browsers comes from ad revenue sharing agreements.

I tried using Firefox. I had it as default browser for 2 years but I just keep going back to Chromium. Firefox is slow and crashes/hangs too much in my experience. It was even very slow to open my automatically generated tables for accounting (for simple html but very big files because accounting regulation in my previous country of residence were brain dead). I don't think often published benchmarks tell the whole story there.

Now I am back to Brave and very happy. Almost no ads, super fast, doesn't crash or hang.