This one was a nice try but carries a lot of old and chrome-specific-stuff around. I've created a new one from scratch just reusing the passhash-algos. The old site-tags can't be imported as the password-API in Firefox is not ready yet.

The new one is using popups instead of reusing the password-field which is a big security issue.

Feel free to give feedback or contribute:
https://addons.mozilla.org/en-US/firefox/addon/password-hasher-ng/
https://github.com/phreaknerd/passhash-ng