If you do not yet have a Matrix account, you can use your Mozilla SSO (what you use to log in on here or on mozillians) on chat.mozilla.org, which will then generate a user on Mozilla’s Matrix home server.
The IRC room is currently bridged to a telegram group. The Matrix room is not currently bridged to that group, though it would in theory be possible.
If you have questions or issues, feel free to raise them here.
Oh, and in case you want to try out another new Mozilla platform where you can log in with SSO, check out the mozilla.ch community group on the community portal: