These are the navigation extensions I currently use, for later reference...
Firefox has memory leaks. You need to restart it once in a while. That's a drag, because you'll lose all the tabs you still had to read. Enter Session Manager, which saves the state of your tabs and restores them when you restart. The default behaviour is to only restore after crashing, so don't forget to customise in Tools->Session Manager.
Keyboard navigation:
I love my keyboard. Mice are mostly designed for leisurly tasks. Some day I'll write a post on user interfaces. And I love Vim. It's the only editor I use, all the time. So naturally when I found Vimperator, a plugin that makes Firefox behave like Vim, I was happy. The plugin still has some rough edges, but it's fantastic, it's really the best thing since Hit-a-Hint (see further).
I also use mozex lets you set external programs for some actions. In particular, this lets you set a shortcut to edit a form's textarea with the editor of your choice (Vim is of course mine). Alleluia!
I used to use those extensions as well, but they are pretty much all obsoleted by Vimperator:
Hit a Hint: select a link with your keyboard. In the process, find hidden links that you'd never see with your mouse. HaH suggests another extention named surfkey, which I'm not fond of. YMMV.
Keyconfig: Change keys. With Functions for keyconfig, you can now use your keyboard properly, and have h,j,k,l navigate in the page.
A note to Firefox developers: get all the plugins to be configured the same way, will ya? Some create a menu entry in Tools, others are configured through the plug-in manager... it's a mess!