As of Firefox 64, the ability to preview RSS feeds natively has been removed, which kind of sucks. I get why they did it though (or do I? Hmmm…). Anyway, to get the functionality back, you need to install one or more add-ons. Since Firefox 63 came out which lost the ability to detect RSS feeds, a lot of RSS feed related extensions have started appearing.
At the moment, I am using Awesome RSS to add an RSS Feed indicator to the Location bar and RSSPreview to preview the feed much like before. For this to work, the RSS option needs to be selected in the Subscribe Using option of Awesome RSS. There might be a better option but these work for now for my needs. For those that use Feedly, Awesome RSS does have a Feedly option as well.
Hopefully this helps anyone that’s missing the RSS functionality. 🙂