We hit Browser compatible issues on a lot of old EL5 Linux Environment.
An alternative is using Firefox ESR version.
Just download RPM. and issue #yum localinstall firefox*rpm
Ref:
http://rpmfind.net//linux/RPM/centos/updates/5.11/x86_64/RPMS/firefox-31.2.0-3.el5.centos.x86_64.html