Rated 5 out of 5 stars
Why isn't it posted in the author's article?
Right click the download button and select to save as.
Unzip the .XPI file.
Edit the Install.rdf file with WORDPAD.
Edit the line about firefox MAXVERSION to a number like 11 or 12, and save it back to the location you extracted the files from.
RE-Zip all of the files back into one zip file and change the .ZIP extension to .XPI .
Install and Enjoy!
(If you need more help doing this, GOOGLE "how to edit install.rdf" without the quotes.)