Brilliant Rated 5 out of 5 stars
This is the coolest add-on I've seen in a real long time. Who uses the arrow keys anyways? And your left hand just naturally sits right there on the keyboard, so it's extremely functional.
The only issue I've experienced so far is the focus sometimes gets switched to something else, so I'll be pressing the S key to move down a page, only to be brought back up to the top again to a refocused search bar with 'ssssss' typed into it.
Kinda annoying (and it even happens on FF's add-on page) but a fix for that, and perhaps some extra key binding options and it's perfect.
5* all the way!