A bookmark doesn't retain where you are on a page. Only the page's location.
Bookmarks also don't relate to your current browser session. I usually organize mine by topic. For current task work what I want is a stack or other less-organized list that I can skip back and forth on.
Firefox's session restore was configurable. I'm a few revs back on Debian (24.4.0), so I'm not sure what all's changed recently.
Perhaps listening to a passage of text and typing some notes and reading a web page belong to a given task, and I might want to stash that away and pick it up later. Although I'm probably kidding myself thinking I can multi-task, and manage multiple tasks, sessions. Freezing one or two though might be useful.
If bookmark management was better in the browsers I'm sure people would use them far more.
You'd almost think the browser developers want you to save all state to their proprietary Web-based silos or something.
I was trying to think what's the difference between a 'frozen' web page / state and a bookmark. There is a difference, but for some pages, a bookmark is enough. I've always thought that tabs are really just perhaps a more convenient bookmark, but they are prone to abuse. Restarting Firefox with howevever many tabs, doesn't now reload all tabs like in the past. You loose state, but they are lighter in weight.