I use the feedbro extension of FF on desktop. Works fine.

I’ve been using Newsblur for years, matches the Google Reader feel pretty well.

May I ask you why you want a web app for RSS and not a native RSS app?

Feeder is my go to but in the last week, someone published the https://rss-parrot.net for mastodon/ fediverse which means that you can include any rss feed. I’m moving to it