Well.. works for me! Rated 5 out of 5 stars
Had my extensions backed-up and reinstalled on a new machine within 10 minutes, however this will be quicker next time as I had to read the FAQ first (I recommend you do the same though the whole thing is pretty uncomplicated).
I plan to keep the XPI file on my pendrive ready for re-installs and setting-up friends machines with my pick of extns.
This peice of software has saved me a lot of time.