"sanctioned" is the term they use to describe the relationship with us, I was just using their terminology, but yes users are allowed to use it to play on official servers.
You can load and get into game with Firefox, you can click the proceed anyway link at the bottom to try it, however performance is worse and some features don't work.
Namely Firefox is missing support for the `FileSystemFileHandle` locking mode which prevents multiple tabs from running (cannot share file handles), and they have a negative standards position on implementing some other parts of the File System Access API like `showDirectoryPicker()`.
I've been working on an WebAssembly Ultima Online browser client. It's a heavily modified port of https://github.com/ClassicUO/ClassicUO (an open-source UO client written in C#).
You can play the test version using Chrome, however beware as it needs to download over 2GB of game resources, and still has a few glitches with audio. That said it runs at a stable 60fps on most systems I've tested so far. We have two test servers, one in AU and EU. Auto-account creation is on so just type in a username/password to get started.
The server the client connects to is another project myself and a few others are working on. It's a ground-up rewrite of an early 2000s custom server for UO called Zuluhotel.
https://uo.com/wiki/classicuo-web-client/