Just Use a Keyword Rated 1 out of 5 stars

You don't need this.
Simply create a keyword like 'goback'
with the url:
http://web.archive.org/web/*/%s

Then just type goback followed by a space and the domain for the site you want.
example: goback yahoo

This will bring up the Wayback machines archive of the site, you can then add the rest of the path for specific page in the native Wayback search form at the top of the page.

Only works for domains Rated 2 out of 5 stars

It doesn't work for addresses with paths, as it turns the slashes into escape codes: (/ turns into %2F), which don't work on the wayback machine.

This review is for a previous version of the add-on (20100602). 

Rated 5 out of 5 stars

Awesome site - this makes it very easy to reference!

This review is for a previous version of the add-on (20100602).