The guy that hosted the original Firefox & Thunderbird icons was caught by mozilla and forced to take them down. I however have put them back up, and was smarter about it, but I wont say how. *cough*fake info*cough* I managed to retain the 2.3 install instructions:
How to install 2.3:
This theme can now be installed by simply extracting the tar.bz2 file into
either $HOME/.kde/share/icons/hicolor or
$HOME/.kde/share/icons/crystalsvg or if you want to install it
systemwide (making it available even to gnome)
into /usr/share/icons/hicolor or appropriate directories in your kde
installation (/usr/kde/3.2/share/icons/hicolor on gentoo).