[arghwhat]

Mostly because it is a lot of work and tends to lead to new and exciting compromises.

This is for example why Chrome decided to fork WebKit.

(I do believe that people messed around with Electron drop-in replacements, but it's probably hard to sell if it doesn't bring anything new to the table.)

[alexlarsson]

It is expensive, but also it is a competitive advantage (of sorts), as we can see with electron. At the end of the day its just a strategic decision Mozilla made to focus on the application itself.