Okay. Here's a weird one...
I've noticed that the Brief RSS aggregator extension for Firefox doesn't work on Fedora 13. After configuring RSS feeds, if I close Firefox and attempt to re-launch, FF will self-terminate before opening any windows. Once this happens, the only way to recover seems to be to launch FF from the command line in safe mode to disable the Brief extension. With Brief disabled, FF launches fine.
I've reproduced this with new user accounts that have no existing profile information so it's not a case of old cruft causing FF to get confused. Likewise, I removed the Fedora-installed version of FF 3.6.3 and all the associated wrapped plugins and installed the official FF 3.6.3 binaries from mozilla.org yet the problem persists.
Now, I've installed the official FF 3.6.3 on my older Fedora 11 system and the Brief extension works fine over there. So the problem seems to be specific to Fedora 13.
Fedora 13 (i686 32bit)
Firefox 3.6.3 (both Fedora-installed and the official binaries from mozilla.org)
Anybody have any clues?