If it also can subscribe to substacks, it's like a Pocket that actually gets updates. Heck, they can just build it on Pocket!
It seems like that could be incredibly valuable for the open web — and a free tool for conveniently reading newsletters built into Firefox might even increase market share? Looks like they removed it in 2018...but I never heard of people switching to Firefox from Google Reader, so I assume it was never very good, or poorly advertised?