@firefoxwebdevs I would've liked to see mobile testing being more reliable and forming part of the main dashboard. It's such a big area to still not be included. I'm aware it's in under investigation, but it's there for a few years now.
If I had to pick one, probably WebNotifications. I use them a lot as a user and there's loads of differences between browsers. None of them work perfectly, but Firefox and Safari in particular are missing a lot of important features like image, renotify, silent and icon (Safari iOS).
https://github.com/web-platform-tests/interop/issues/1047
https://developer.mozilla.org/en-US/docs/Web/API/Notification