I 
Firefox, and I use it as my daily browser. But unfortunately it has been a little slow to adopt web components, the cutting-edge tech that underlies the new SC. Not their fault, I hasten to add; it was an initiative of the Chrome team, and FF has been implementing it since it was standardized late last year (unlike the browser of a certain major OS manufacturer).
We can expect full web component support in FF 63 later this year.
Meanwhile, SC works on FF via what is called a “polyfill”, i.e. an extra bunch of code that more-or-less does the same job, just slower and worser. But don’t despair: you can enable web component support in FF if you’re willing to get your hands dirty. Here’s how.
- Type in
about:configinto your address bar. It’ll bring up bunches of options. - Type “webcomponent”.
- Double click both
dom.webcomponents.enabledanddom.webcomponents.shadowdom.enabledto enable them. - Drink mead and frolic!
This should fix a bunch of annoying bugs on in SC and make the whole experience smoother. If it’s still not 100%, well, that’s why it’s not enabled by default just yet.