Bookmarks and tabs are far from being easy, solved problems. For example adding a Firefox addon to display tabs vertically in a tree structure instead of a horizontal list has changed the way I use the internet (and I have always used a lot of tabs).

In the end tabs are just like booksmarks, except that they keep state during a browsing session. I find it very weird that we keep those very similar concepts in completely different UIs. There's a lot of room for improvement in that area.

They are "in" there, but they are very far from being perfect. Managing bookmarks is a constant pain, and most browsers don't even offer things as simple as watching if bookmark has gone bad, let alone any added-value services. Tabs are not spectacular either - I regularly keep dozens of tabs open, and no browser I've seen handles this scenario very well, most just show something like "ok, you've got a lot of tabs, guess which is which". Handling them better, separating by importance, supporting grouping etc. - haven't see it anywhere. I'm sure there are also other places browser experience could be improved.