Could you please post the output of?:
$ ls -l /usr/share/icons/ | grep Tango
If none, you have to download the Tango icon theme; to do so in debian, ubuntu, mint or any other debian-based, run:
#aptitude install tango-icon-theme
If it was installed, try running
#aptitude reinstall tango-icon-theme
Depending on your distro you'll need the appropriate package (.rpm,.deb).
You should also be able to manually install it getting the tar.gz file for the icon theme.
Alternatively you can configure firefox to use another icon theme.