I'd guess episodic and spacial memory help. All the tabs I'm likely to access together are close together, & were opened at similar times. (So, you can remember what the page was about without needing the full title to associate the tab with the contents).

I probably wouldn't have more than window-width of tabs open at once in a single window, and no more than 2-3 windows open at once.

A tree is a nicer structure for the "where did I open this from", though; especially if the screen space is available.