I am curious. Thank you!
This is what they say in their FAQ
Why aren’t you using Mozilla’s Gecko engine on laptops?
We were, under a partially sandboxed, multi-process architecture called Graphene. But we did a careful head-to-head comparison and by every measure, Electron/chromium won. We wish Mozilla well, but as a startup, we must use all sound leverage available to us. For web compatibility and in particular Chrome compatibility, this means chromium.
Wow, interesting. Thanks for sharing!