Enable to change tab by "gj" and "gk" in Firefox with vimperator
Make also "jj" to behave as "<Esc>" in Vim

Only "kk" is not enough.
When inserting a word which ends with "k" like "hook", input

i h o o k Esc

but now Esc is realized as "k k", so input

i h o o k k k

in this case, when inputting 2nd "k", insert mode finishes and the result will be
"hoo".

So, because another key binding is needed, remap "jj" to "<Esc>".

Enable to input

i h o o k j j

to write "hook" now.