Just some data for you that might help...
If I have Firefox set to use FoxitReader or acroread (Edit->Preferences->Applications->Portable Document Format),
the pages display fine. On the other hand, just like you said, if I have that preference set to "Preview in Firefox",
then I also get the gibberish you reported.
I'm not willing to guess what it means, but it's definitely not just happening to you.