you might want to start by INSTALLING the current and supported version of firefox
the old ff 10 is unsupported
i have FF 15.01 running just fine on ScientificLinux 6.3 32 bit
do you have THE SAME rpms ( the EXACT same) installed on both???
for video you DID install and configure "yum-priorities"
and you installed the video rpms from rpmforge ( rpmforge-extras can be incompatible with red hat upstream)
Both systems have the current Flash plugin installed
from adobe web site???
or using yum ????
is it the 32 bit flash ?
or the 64 bit flash?
is Firefox the 32 bit version?
or is it the 64 bit version?
On my SL6 install on 64 bit hardware FF 15 64 bit
and the 64 bit FLASH
run just fine
once i removed the wrapper and links ( alternatives)to the 32 bit flash