Yes, a modern browser should be the cornerstone of any mobile OS. My N9 was incredibly useful for many years after it was abandoned by Nokia because a fresh Firefox Mobile was available at OpenRepos. This, together with excellent online/offline navigation and a terminal, was all I needed.
SFOS is in a much better position as it has many apps, along with an Android compatibility layer. However, it is lacking on the native browser part. Personally, I think maintaining a modern browser is a huge task and Jolla doesn’t seem to have enough resources. Firefox has some old builds for Mer, it should not be a lot of work for Mozilla to get it working.