I just ran into Vimperator today. It's a Firefox add-on that enables VIM-like functionality directly info Firefox! I installed it to see what it's like and it seems pretty interesting (though not sure whether I'll get used to it or no).

By default, Vimperator hides the Firefox menu and toolbar, but many common Firefox keybindings still work.

As you would expect, you can open a site on the current tab using the :e command (tab will present a list of candidate URLs based on what you've typed so far). Using :open or o/O also works, and t/:tabopen can be used to open a URL in a new tab.

You can also close the current tab using :bd. Also, switching between tabs can either be done Ctrl+Tab/Ctrl+PgDown/Up or using :b[n]. where [n] is the tab number.

The usual Vim navigation keys (hjkl) can be used for scrolling around the page, among

Overall, seems like a pretty nifty add-on, so I'll try it for a while to see how it goes.

Update: Couple of things I've noticed to watch out for:

1. When completing URLs to open, the order of the entries in the list seems to be pretty random, instead of how Firefox usually does it (most used entries listed first). This can be a bit annoying.
2. The new modes do block out the GMail and Google Reader key shortcuts from working. To use them, you first need to press I and then they will work normally (press Esc to go back to normal mode).

Firefox, Vimperator