Months later, inbox.google.com still requires Chrome, despite it working perfectly well in Safari when spoofing the UA, and claims that it was held back by "performance issues on some animations", and while I've had a few occasional bugs running it in Firefox, it seemed to be altogether quite fine. Anyway I stopped using the thing a month ago because I simply dropped Chrome†.
† slowly becoming too much ChromeOS-and-why-not-a-browser (including terrible memory use) extending its tentacles in ugly ways into my desktop of choice.