This was tough for us, because we're big believers in the standards-based web. We 100% promise that all the code we generate works across all browsers.
Unfortunately, while most of the editor works in all browsers, getting full support is really hard. We're doing a lot of things that get pretty deep in the browser, so we wanted to lock things down as much as possible.
We almost were electron-only to ensure consistency, and can still give you the electron app today. We felt it was better to launch with Chrome + Electron than just Electron for an optional installation-free experience, which is why we only support Chrome.
I'm personally really sorry as an engineer that we don't support Firefox, Safari, and other browsers yet. It's on our roadmap :)