We did have people developing a new browser engine. It was called Servo and Mozilla went and killed the project.

Maybe it just wasn’t going anywhere and they didn’t think it was right for Firefox, maybe it was experiment for experiment’s sake, maybe it was always destined to be a side project. I don’t know.

Servo was an experimental browser engine for Mozilla that they were using as a testing ground for their investment into the Rust programming language; the project did bear fruit as components were slowly integrated into Firefox starting at version 57 with Project Quantum. They did eventually complete that transition and everyone using an up-to-date version of Firefox is using various Servo components (I'm personally a big fan). Unfortunately, when COVID devastated the economy Mozilla axed some of their side projects, which included Servo itself. Maybe we'll see further investment in the future, but they did already do what they wanted to with it for the most part.