I'm not making it up. I don't remember the exact details – you'll have to look at the archives of the Mozilla dev-platform mailing list if you want them – but I seem to remember the usage was below 0.001% of users.
I personally read RSS with Thunderbird, which is a much better experience anyway.
> And even you premise was true (which is debatable), it cost absolutely nothing for Firefox to keep a functional RSS reader without putting much effort into it (or instead, you know, putting efforts in pushing ads to people who never wanted them).
It actually does. Every piece of software that you need to maintain is a tax. Paying the tax makes sense if the code is useful, not if it isn't.
Also, frankly, this implementation didn't bring anything to users. It wasn't nice to use. There are much better RSS readers than a browser – starting with RSS WebExtensions.
Additional features that the average user does not want to use or care about creates confusion and a feeling of bloat. An official extension would be nice though.
You'll need a source for that, since most Wordpress sites which constitute a HUGE part of the web have pretty much by default rss feeds. It's not because you are not using it that nobody is.
And even you premise was true (which is debatable), it cost absolutely nothing for Firefox to keep a functional RSS reader without putting much effort into it (or instead, you know, putting efforts in pushing ads to people who never wanted them).