I think it's the sort of thing where this works well enough for the same purpose that other people use a read-later or bookmarking system for, and it's reliable enough that people don't care to switch (or don't know there's an alternative).

It's good that Firefox is trying to adapt to the way people use technology, rather than trying to force people to adapt to how the technology works - I think that's a much faster road to widespread adoption, and it creates more goodwill on the part of their users.