Well, for those folks who have turned up this thread in a search, and were perhaps hoping for something slightly
more helpful than "upgrade your OS, you moron!", I'd like to report that I finally figured out how to do this. Perhaps this will be helpful for other folks as well.
First of all, I wasn't able to find a way to do it with yum. I'm not sure why, but really all the yum is doing for you is working out the dependencies, and I was able to do that by trial and error. So this should work fine.
Fire up your Google and run two searches:
rpm "xulrunner 14" "fedora 15"
rpm "firefox 14" "fedora 15"
This should turn up several links. Make sure you find the one that a) is for your architecture, and b) actually links to an actual RPM. Also, substitute the version of Fedora you have if it's not 15.
If you're not sure what architecture you have, try:
winterfell /var/install # uname -i
If you're not sure what version of Fedora you have, try:
winterfell /var/install # cat /etc/fedora-release
Fedora release 15 (Lovelock)
If you have x86_64 architecture (like me) and you have either Fedora 14 or Fedora 15 (like me), then I can save you some effort:
Download the appropriate two RPMs somewhere. Now, just pop into a root window and type:
yum upgrade libvpx
rpm -Uvh xulrunner14-14.0.1-1.fc15.remi.x86_64.rpm
rpm -Uvh firefox-14.0.1-1.fc15.remi.x86_64.rpm
Again, substituting the appropriate Fedora versions and architectures ... basically, just run "rpm -Uvh" on the two files you just downloaded. But upgrade libvpx first, or it'll whinge at you.
Hope this helps some folks!