Honestly the RSS support in Firefox and friends was just never that great. A long time back I used Sage and loved it, but when Firefox moved to use standardized plugins it wasn't an option anymore.
Mozilla throwing out their entire ecosystem of add-ons (like sage) so they could copy chrome's web extension instead of continuing their own far more powerful extensions was another of the indicators that Firefox is no longer a browser first. It is now a secure JS engine and DRM video first application for commercial transactions. Any ability to be a browser comes after those requirements are met.
So I built Brook, it's pretty simple and hangs out in your sidebar on Firefox: https://github.com/adamsanderson/brook