I managed to quick fix this on my own by patching /usr/local/lib/firefox3/chrome/classic.jar.
Steps (as root):
- unzip /usr/local/lib/firefox3/chrome/classic.jar
- apply the attached patch
- zip the 'skin' directory again as classic.jar
It seems that the icon file names have changed between gtk-2.20 and gtk-2.22. As far as I understood the real file names should not bother applications and I only have this problem with FF.