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) :)

As it always had to be, good job!

This is exactly what I'm looking for. Thank you!

Does exactly what has been removed in Firefox Quantum version 64.0 (dec. 2018): previews RSS feeds (ie detects when you open one and uses an appropriate style sheet). This is minimalist and exactly what I was looking for. Thanks a lot for sharing this!

Thank you for making this, I'm so excited to use it!! I can't figure out how though, can someone please explain? I'm used to seeing RSS from my "Bookmarks toolbar", I don't know where to go for this one.

It does what it advertises, re-create the old Firefox feed preview layout.

Thanks! I was really beginning to miss that built-in RSS feature in Firefox. This is an extremely helpful replacement.

Longtime hardcore RSS user here. This extension does basically *exactly* what I expected it to. Thank you most kindly for giving back what Mozilla has taken away from us!

I'm sure my experience is not anything special, but it can't be repeated enough: THX Aurelien for compensating for Mozillas brilliant dev strategy. I guess the flat RSS code doesn't pay their bills and Pocket ( and Cliqz, and..., and...) does. Pity you had to spend your valuable time on it, and THX again you did...
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For people searching to address their RSS needs: this addon BRINGS BACK the ability to view RSS feeds like you were used to do before Mozilla decided to remove it with version 64. It works brilliantly in 99% of all feeds I follow. Have to admit that there are some "news" sites, where you can encounter a "Error loading stylesheet" [ == .xsl ], but reviewing code that seems to be more the fault of these (dare I say?) amateurish news organizations than of this addon.

A tidy replacement for the 'View RSS' feature that Firefox killed. Excellent work.

Even excellenter if it could duplicate the 'Subscribe to feed with... [external reader]".

In the meantime, I'm very impressed with the developer for making this available. My thanks.

Nice to have something close to what Mozilla destroyed. Thanks!! Hint to Mozilla, I really want Firefox to succeed, but you're making it hard.

This is a perfect substitute goods for removed default reader in FF 64.0!!!. I had looked for this simple and exellent add-on for 2 days

Thank you David! An efficient and essential extension for radio collectors like me!

"Awesome RSS" + "RSSPreview" is a great fix for Firefox 64+.

Thank you, Dev! The extension is working very well so far. I wasn't sure I was ever going to find a Feed Parser just for the xml, rss, atom files I directly link to.

Most other extensions include a complete feed management solution and I wanted to avoid bloating Firefox. This extension responds to a very specific issue with FF 64+ inability to parse rss feed files directly. Great Job on this - so very timely!!

This extension does exactly what I wanted it to do. When I now click on my RSS feed it opens up seamlessly just like it did in FF version 63.0, before Mozilla removed the built in RSS reader. I am very happy with this extension. It does not add a lot of bells and whistles that I do not need or want. Thank you very much Aurelien.