Turns out bookmarks are still there.

Add a bookmark via overflow (three dots) then star at the bottom.

Access your bookmarks via overflow (three dots) -> Your Library.

I'm not really clear why it has both bookmarks and collections.

Edit: how to add a bookmark changed from the bottom shelf to the overflow menu with today's update

I think they're just trying it out as a feature for now. It's a Preview browser so some things won't work 100% (for example a bug[1] is still present in 2.3.0, the latest stable version, requiring me to switch browser for file uploads.)

[1] https://github.com/mozilla-mobile/fenix/issues/6279