How do I change the Firefox 23.0 icon back to the old one?
Download the Firefox 22.0 release binary from one of these locations:

http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/22.0/mac/en-GB/
ftp://ftp.mozilla.org/pub/firefox/releases/22.0/mac/en-GB/

Double-click to mount the Firefox 22.0.dmg file.
Copy the icons using Terminal:

cd /Volumes/Firefox/Firefox.app/Contents/Resources/
cp document.icns firefox.icns /Applications/Firefox.app/Contents/Resources/

Reboot.


Note: If you want to do the copy from Finder, you'll need to right-click on the Firefox.app icon in the mounted dmg file and select "show package contents" then using a separate Finder window do the same thing for /Applications/Firefox.app, then change into Contents/Resources in both windows and copy the files across.