Unsupported Firefox addon
I use the Nightly Tester Tools, instead of tweaking the XPI file: https://addons.mozilla.org/en-US/firefox/addon/6543?src=api
This allows you to mark an addon as compatible (something like that), which is pretty much a necessity for nightly build testers. You should use it with caution, of course.
Try scrolling down the addon page to the bottom, there is a link named "older versions". If you click this link you will be taken to a page which lets you download all versions plus the latest.