Miv by Mathias

The add-on has some Firefox navigation keymappings inspired by Vim:
j : down
k : up
f : scroll down
b : scroll up
gg : scroll to top
G : scroll to bottom
Shift + ss : change current tab to https://duckduckgo.com
Shift + xx : change current tab to https://google.com
Shift + hh : go back
shift + ll : go forward