Firefox is a legitimate alternative, it works for >99% of the world's websites and is an actual alternative i.e. not a fork of Chromium. We need to support Firefox. I've been using it for nearly the last 5 years full-time and have never needed to open Chrome.
Do some Google websites perform worse on Firefox? Sure, but the entire reason that's because people don't use Firefox enough i.e. some websites are worse on Firefox. It's a cycle. People don't use Firefox enough, which is why some websites are not performant on Firefox and so on.
Firefox expanded and became famous because of a large amount of developers telling their families and friends that it's the better browser. Same was the case for Chrome. This needs to happen again.
As a side note, can someone at Mozilla please make it so that I can donate directly to Firefox? Is that not possible for some reason? If I knew that Mozilla wasn't going to spend my money on some stupid new idea, and it would directly to supporting Firefox development, I would defintely pay a subscription each month to Firefox, and feel like a lot of other people would too.