Archeosfox is a project mainly written in JavaScript, it's free.
ArcheOS Firefox extension
Instructions on how to build archeosfox
PREREQUISITES:
* devscripts, mozilla-devscripts, git-coreBUILDING
* git pull
* cd archeosfox/
* ...do your modifications here...
* update debian/control
* update debian/changelog
* git add .
* git commit
* debuild
* debuild clean