Firefox icon disappeared after the latest update
After the latest slackpkg upgrade-all on Slackware 14.0 the Firefox Icon disappeared from the menu on XFCE. There was no effect on KDE.
This was solved by:
# cd /usr/share/pixmaps/
# ln -s /usr/share/icons/hicolor/256x256/apps/firefox.png .
# update-desktop-database -q /usr/share/applications
Log out and log in back to XFCE.