There were some nice Firefox extensions that supported bookmarking and reloading tab trees. Bookmarking a tree would result in folders, so the whole structure was preserved.

Bookmarks can definitely work, but yeah, a flat list can't. I think a better approach would be something like a mind-map.

Bookmarks have a great ux for it: save all open tabs to a bookmark folder, open all bookmarks in a folder

And then what happens to the trajectory of clicks that I followed to land on the page that was open in the tab ?

Rhetorical question of course. With an open tab I have that information and can go back and forth up the path. Its very useful context to have when I revisit the tab. In my line of work I research many things every day. Firefox open tabs have been a godsend. I literally have 900 to 1200 open at any given time

The browser ought to remember the history of a tab when saving that tab into a bookmark folder (think VM save state).

Photoshop has work flow like this - you can split the undo history into a tree like structure (to a degree, it's not infinite).

+1 indeed