Unlike most open source code that's published online, it's damn near impossible to just go an re-implement your own browser if all available ones are not to your liking. Up until somewhat recently, Firefox has been the best browser for tech minded and privacy focused people, but it seems like Mozilla has lost their focus and are implementing features that their user base find questionable. I'll admit that some other features are very welcome, like containers and the like. But I do understand that their core userbase feel disappointed and whenever they see their best choice in browsers move all the more closer to the other subpar choices.
Specifically, people feel it was a let down to throw away the layout engine that was rewritten in Rust and drop the efforts to modernize other components too. There's a lot of people who feel let down with the deprecation and removal of support for older, more powerful plugin APIs.
And as for writing a browser on your own, lots of competent developers might implement an HTML renderer. I don't think a single person could implement a browser that's capable running youtube, banking websites, amazon, a fully features javascript VM, all the modern sandboxing and various web APIs (that now even include Bluetooth and USB).
I don't think bringing pitchforks to Mozilla will make them write better software or listen to users more, but I don't want to de-legitimize the concerns that some of their core audience might have. IMO, Firefox is still the best browser to use. Chrome and it's derivatives, please go.