Mozilla removing long standing support for this from Firefox was a clear signal what their browser is for now and what it isn't for. Firefox is for running javascript applications and consuming DRM video and visiting your bank's website, just like chrome. It is not a browser for surfing the web and looking at websites.
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.
Correction, Firefox is controlled opposition for Google to deflect from being accused of having a browser and browser engine monopoly. Mozilla post 2011 is a very different beast; they decided that copying Chrome was the way to go, stripping out all useful features culminating in the removal of the powerful XUL addon system citing security problems (while ironically the amount of malware extensions shot up AFTER they switched to the Chrome originated Web Extensions format, which are far less limited in capability). Meanwhile, the simple minded continue to fall for their claims of respecting privacy and user choice.
