did you fallow the instructions on the wiki?
mind you on the OLD centOS 5.8 ( the ONLY 5 version that is supported )
assuming that this is CentOS 5.8 do to the .el5 in the post
if this is in fact RHEL 5.8 then see the RHN knowledge-base on the red hat site- the FULL instructions are there
yes firefox 10 AND NOT 15 is used
the java 1.7 might not work well on the old firefox10
i would use the yum repo install for java then run
( see the Oracle java web site , download page)
that will make sure that you have a version that is compatible installed
as to the plugin
that might be fun
it is called " alternatives"
has a link for java
BUT to the 32 bit version in /etc/alternatives
that in turn is a link to /usr/lib/java/???????
and someplace in there is the java.so
it is a very round a bout mess to use a 32 bit plugin ( might be the 64 bit ? unknown ) on a ? 32 bit firefox on a 64 bit OS
--- it is a MESS ----
one that i removed on my ScientificLinux6.3 install
by installing the custom 64 bit firefox 15 build
and using the 64 bit java and flash plugins