the function check-gre in contrib/run-conkeror does a grep on the gre.d/1.9*.conf files,
grabs the first result and hopes it somehow works out.

It doesn't on my ubuntu system, where:

%ls /etc/gre.d    
/etc/gre.d/1.9.0.1.system.conf  
/etc/gre.d/1.9.0.3.system.conf   
/etc/gre.d/1.9a8.system.conf     
/etc/gre.d/1.9b3pre.system.conf
/etc/gre.d/1.9b4.system.conf
/etc/gre.d/1.9b5.system.conf
/etc/gre.d/1.9.system.conf

My current xulrunner system is 1.9.0.3, which doesn't sort on the top.
function check-gre can me modified to select the most recent .conf file, using
a simple rewrite as.

* Current check-gre
function check-gre () {
    grep -h GRE_PATH  $(grep -l '\[1\.9' "${1%/}"/*.conf) | cut -d= -f2 | head -n 1
}
* New (proposed) check-gre:
function check-gre () {
    grep -h GRE_PATH  $(ls -t $1/1.9*.conf | head -n 1) | cut -d= -f2 | head -n 1
}

Hope this helps.
Now, I'm able to upgrade my xulrunner w/o conkeror failing to start.