Do you mean sandboxed?

Unfortunately Mozilla is going the enshittification route more and more. Or good in this case that the Firefox Phone did not take of.

This comparison looks neutral: https://www.freie-messenger.de/en/systemvergleich/xmpp-matrix/

I use the Nextcloud news app which is a RSS feed reader. But mainly because I run a Nextcloud instance anyways. For news only I would not install Nextcloud - too much overhead.