Whatever you decide to build into the desktop version, as much as possible should also be included in mobile versions, hence I’m not going to name individual features.
The key to a good product is good user experience, and that is achievable through consistency.
If you manage to build a product that will work, look and feel nearly the same across platforms, you’ll manage to win over many users even from the most popular browsers.
At the moment, Firefox is grossly inconsistent. Read later is on iOS but not on desktop. Why? Translation achievable on desktop but not on mobile. Why? I can pin sites to favorites on iOS but not on desktop. Why?
The list is long. You get my point. Just don’t create discrepancies.