As you suggested, muungwana, I finally reinstalled PCLinuxOS from 2010.7 ISO, it took me some time but it was worth it, now firefox works fine !
Thanks again to all of you, it was my first post to the PCLinuxOS forum, I've been very happily surprised with so many people trying to help me. One more reason to stick with that great distro, the most user friendly I've never tried.
Regards,
Denis