Firefox Quantum [?]. I've been using the nightly builds for some time, it's renewed so much hope I had for Firefox beating the "but Chrome is faster" myth - it only feels faster. Mozilla overhauled the UI to be just as slick. I can't wait to see Firefox in 2018 now they're working on "HolyJit" which is a new JS engine, which, just like Stylo, is written in Rust! Safer and faster than Chrome in the long run, that's my bet.
Before Electron we had XULRunner, so websites-as-an-application running on Mozilla code could make a comeback. I'd be willing to bet Servo+Stylo+HolyJit would be much better than Electron. You know, it's funny that Atom is slowly rewriting performance sensitive parts in C++ blog.atom.io/2017/... so maybe this whole trend will just go away eventually?