I was so happy to pass by RSSPreview, which seems to be _exactly_ what I was looking for (a very simple RSS feed viewer, without tons of features that are unnecessary to me)!
But... the main reason I've been looking for a replacement to what-Mozilla-removed-from-end-users-recently is (French radio) France Inter's numerous feeds. I was still confident when I saw RSSPreview display a page like http://radiofrance-podcast.net/podcast09/rss_10217.xml the same way FF used to display it.
Unfortunately, my first attempt to download one of the available MP3 podcasts led to an immediate error message (here translated from French):

"Download error"
The download can't be saved since an unknown error occurred.
Please try again.

French message:

"Erreur de téléchargement"
Le téléchargement ne peut pas être enregistré car une erreur inconnue est survenue.
Veuillez essayer à nouveau.

Could dev/author Aurelien David take a look into this?
I'm ready to raise my notation greatly as soon as France Inter is correctly supported :)
Thank you!

Update 1: thanks for getting in touch Aurélien! Considering that it works for you, I'll continue my investigations (I'm running FF 65.0 on a GNU/Linux system, RSSPreview 3.2). It could be that another of my extensions conflicts with yours... Sure, in case I'm not able to solve this by myself, I will open an issue on Github.

Update 2: CONFLICTING extension found -> uMatrix! Disabling the latter solved the issue - surprisingly, since it wasn't blocking anything (nothing detected on this radiofrance-podcast.net XML page ^^). Anyway, despite of the uMatrix icon displayed in grey color for radiofrance-podcast.net, I disabled all uMatrix filters for this domain (On/Off button) and got around the problem that way. But still wondering why/how uMatrix is impacting RSSPreview...

Update 3 (FINAL): the "problem" actually comes from uMatrix standard configuration, which blocks any media not belonging to the visited domain (radiofrance-podcast.net in my case). Podcasts are actually hosted on rf.proxycast.org, as I could read on the uMatrix logger-ui.html page (accessible via uMatrix "Aller au Journal" top-right option). Allowing media coming from domain rf.proxycast.org solved the issue (without having to disable uMatrix for domain radiofrance-podcast.net) :)