No problem. You can verify the feed here:

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fsocial.network.europa.eu%2F%40EU_Commission.rss

so it definetely is fine. Good luck.

Oh wow ok. So ios firefox is webkit based? Weird stuff.

https://social.network.europa.eu/@EU_Commission.rss works fine for me. What reader are you using?

does firefox not have an IOS version? It might not have an addblocker out of the box but, in android at least, it supports addons and thus ublock origin.