It's a Firefox plugin only, but makes it behave a lot differently - adding vim commands to it. This is not meant for the casual user, but somebody who enjoys Vim, will probably enjoy Firefox a lot more when using this plugin.
Favorite commands include ":hardcopy" for the print dialog or the navigation in general.
If you test want to test it, please start with using the 2.0 betas, which are stable already and provide a lot of improvements since 1.2 (current stable).
I've even written my own command already, which allows me to easily switch between development and live server.
After all, it's really powerful and lets you avoid using the mouse even during surfing.. ^^
The help/tutorial is always available with ":help" - and you can still use the mouse (and normal shortkeys, if you use bypass mode (Ctrl-Z / Ctrl-Y), so there's no real excuse after all..
Update: Vimperator is available in Debian/Ubuntu as "iceweasel-vimperator", although currently only as 1.2-1 - while a few days ago 2.0 has been released and provides a lot of improvements and fixes.
As great as the Debian/Ubuntu package archive is, for plugins like the ones used with Firefox, I prefer the official repositories to (automatically) get any updates.