I completely agree. I never actually used Firefox as an RSS reader, but I did get my mom started on RSS through Firefox, and later she moved on to a dedicated RSS reader. I don't remember, but I think that Firefox would offer to subscribe to a feed via Thunderbird if you didn't already have one set up.
But I think Mozilla's abandonment of Thunderbird is very much in line with their abandonment of RSS, and their loss of commitment to an open web.
Unfortunately Mozilla has dropped thunderbird almost completely so that’s not been a real option either.