Do *NOT* extract Rated 5 out of 5 stars

When you get the XPI, then (we'll use 7-Zip as an example) right-click on the file and select "7-zip > open archive". When you see your list of files, right-click on the rdf and select "edit". Find where is says MaxVersion and change that to 8.0* and save the text. 7-Zip will tell you it's been updated so tell it yes. Then just drag the xpi into Firefox. Should then install for you properly. :-)