https://bugzilla.mozilla.org/show_bug.cgi?id=429903 This is a pretty high profile bug (declared a blocker upstream with a heavy heart by Mike Shaver) causing xulrunner based apps like Epiphany and Firefox to hang with 100% cpu usage and doing so for a lot of people. A fix has just been posted to the original bug but not checked in yet but it looks fairly safe (one-liner) and should probably be included in the F9 release if at all possible.
added to xulrunner-1.9-0.56.beta5.fc9
Fix is available in rawhide, and I can't reproduce the problem with any of the sites mentioned in the upstream bug.