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.