Like the bookmarks and history manager. There isn't anything particularly wrong with these data-table windows, but I don't really enjoy using them either. In some ways I think they should be at the heart of the browser.
I think a lot of people use tabs because bookmark management is so crap.
The only bit I resonated with was the similar sites suggestions. But you'd need a setting to setup suggestion services. There's a privacy concern with that.
Other helper features:
Pagination buttons were built into Opera driven off the rel=prev and rel=next, link elements. Navigation could further be ripped out the page window into a browser control. Searching sites and pages could be friendlier. Better form helpers needed. A good feed reader would be good. Tools to help read web content more simply (readability style) would be nice. Plus I like personalising the look and feel of my web browser ever so slightly, and even Firefox doesn't do that particularly well (it ignores some of my desktop theming).
So I think Firefox should really be thinking how to answer the question: 'How can we make it easier for users to consume web content?'. This has to go beyond the rendering engine. So actually a fatter featureful browser I think would be better - but with some very intuitive and simple controls.