Rated 4 out of 5 stars

Note: To make it compatible with the new 3.6 Firfox

1- Download the Addon (do not install...just download to your computer)
2- Open & extract files in iraniancalendar-3.1.20081222-fx.xpi using unzip/unrar utilities
3- Open install.rdf using any text editor
4- Change the maxVersion to:
em:maxVersion="3.6.*"
5- Save the file
6- rezip the edited iraniancalendar-3.1.20081222-fx.xpi and then change its extension from .zip to .xpi
7- Uninstall the old Iranian Calender and install this new one by opening the xpi with Open File (in FF menu)

I have uploaded this edited file here if anyone is interested. I changed its version number to 3.1.20081222b

http://rapidshare.com/files/352213398/iraniancalendar-3.1.20081222b-fx.xpi

Good luck