I thought you said it was web-based?
They mean it when they say "User ID can only contain characters a-z, 0-9, or '=_-./".
Is that really so esoteric?
Cannot reach homeserver Ensure you have a stable internet connection, or get in touch with the server admin
https://chat.mozilla.org/#/register
It's trying to make a GET request to this URL and 404'ing:
https://chat.mozilla.org/config.chat.mozilla.org.json?cacheb...
That brought me to this page:
https://mozilla.modular.im/_matrix/saml2/authn_response
Which only displayed the word
ForbiddenMeanwhile gomuks also exists and is usable and not abandoned.
On GUI Desktop, various clients are in active dev - e.g. Fractal for GTK/Rust, Quaternion for Qt/C++, Nheko for Qt/QML, etc. They are not as complete as Riot yet, but I'm sure PRs are welcome...
... although if I'm not mistaken, unfortunately none of them supports Single Sign-On yet, which is required for joining the Mozilla instance.
Also straight from the readme:
"Basic usage is possible, but expect bugs and missing features."
Not very promising.