Ok my experience with firefox not loading so far has kinda revolved around upgrading xulrunner to version 1.9.0.6-1. I think it's an upgrade from the pclos big update, but either way as soon as I upgraded it, I could no longer get firefox to load. So I downgraded xulrunner to 1.9.0.5-1 & now I can get firefox up and running again.
You can try downgrading these packages by using the force option in synaptic.
You click on package in the toolbar of synaptic, and then select force version. Then from the pop up window, select the 1.9.0.5 version. You'll want to have the package highlighted that you want to force a version for.
Also you'll have to do the same for all three xulrunner packages
xulrunner
xulrunner-devel
xulrunner-devel-unstable
Also for those of you that are not comfortable forcing the version, or cannot force the version for whatever reason. Then I put together the xulrunner rpms for version 1.9.0.5 and you could wait for Anurag to put them into the repo, and then go ahead and install them.
good luck.