fc5 firefox won't open pdf: Unhandled MIME type: 'application/octet-stream'

[sean]

Since upgrading to fc5 firefox won't show pdf docs. I can 
save the doc and open it in acroread, evince, etc.

When firefox tries to to show a pdf file I get an error window:

Unable to open document

Unhandled MIME type: 'application/octet-stream'

I've messed with mozpluggerrc to use acroread, then I tried 
evince. Same problem.

FWIW, x86_64.

Any help appreciated

sean