Firefox is fine as daily browser and the few websites that don‘t work in it start working if you enable the chrome mask plugin for them.
Before anyone had ever heard of Electron, you used to be able to embed gecko into other apps and it could render html in other applications (that’s how Firefox itself started cause the old monolithic Mozilla suite separated the engine into gecko so other apps could use it, and then a bunch of Mozilla engineers started developing a new UI around it as their side project). Of course they gave up on that, and look where we are now.
Testament to its engineering team really that despite so many own goals and false directions from its leadership (Remember Firefox OS?) the project is still alive.