They sat on this since at least 2016
Delete all data from the previous account and make a new one. Report the troll to moderators. Consider never visiting that place again if the mods won’t help you with the harassment.
If the troll doesn’t have access to the forum’s server, then all you have to do is learn some opsec and be aware of what you share with strangers on the Internet.
If the troll does have access to the server and you absolutely need to visit the forum (which I’d advise not to in such a case), then in addition to what has been said above, use VPN with a hardened browser or Tor to access the forum.
This is not the place to ask Andy Yen questions. Use this form instead.
This is not the place to ask Andy Yen questions. Use this form instead.
Neither Matrix nor XMPP are software, those are protocols. Alsp, there is no point in specifying that XMPP is encrypted without doing the same for Matrix, since you can exchange plain text messages there too.
As for the actual question, here you go: https://www.privacyguides.org/en/real-time-communication/#criteria
There are tools for viewing source code of Firefox extensions, for example this one, which is web-based:
Will delete your account due to inactivity. Could ask for a phone number in some cases