Karl Fogel at
Internet Archive post about the Firefox plugin for their Wayback Machine links to the Mozilla Add-On page for that extension, but the link is stale:
http://blog.archive.org/2010/06/08/new-firefox-add-on-for-searching-the-wayback-machine/
Nobody's fault, and I'm sure I can find the new version. It's just a little too perfect, that's all :-).