I'm was looking for a replacment of Sage++ and found Feedbro to be the best alternative. A sidebar and a more lightweight looking design would be cool. My problem right know with Feedbro is that new articles are not listed (since update to FF 60.0b5 (64-Bit)!? I don't know). My news feed is zero and I'm sure that there are new articles. The manual refresh isn't working either. I got the pop-up that the scanning for new articles started scanning but nothing is found.
Sounds like your Firefox profile has been corrupted which breaks IndexedDB functionality (which then prevents Feedbro from saving articles). You can fix your profile by opening URL about:support and then press "Refresh Firefox". Warning! This will remove your extensions and their settings so backup them first if you need to.
