Firefox -Could not find compatible GRE between version 1.9.2.7 and 1.9.2.7

Trying to start firefox on a new system (Rocks 5.3 running Scientific Linux SL 5.5 <=> Red Hat RHEL 5.5), I got the not terribly helpful message "Could not find compatible GRE between version 1.9.2.7 and 1.9.2.7". I found the answer by a roundabout route, but in hindsight I should have attacked this with
rpm -qa | grep "1.9.2.7"
which reveals the only package at this revision level to be xulrunner. Doing yum list all xulrunner showed that only the i386 version of xulrunner is installed. This is a 64bit system, with both i386 & x86_64 firefox versions. Lo & behold, yum install xulrunner installed the 64 bit version as well and the problem went away.  I guess yum erase firefox.x86_64 would have worked too.
No prizes for the error message though...