FireFox issue with concrete5.7

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I get an error (only in FireFox):

An unexpected error occurred:
"session_start() [<a href='function.session-start'>function.session-start</a>]: open(/tmp/sess_e79da0705e8b5240f9f57f275bcaab64, O_RDWR) failed: Permission denied (13)"

An unexpected error occurred:
Unknown: open(/tmp/sess_e79da0705e8b5240f9f57f275bcaab64, O_RDWR) failed: Permission denied (13)

Clearing the cache in Firefox fixes the issue, but why is this happening? Is this a bug? If a client get's this error, I'm sure I'll have hell to pay.

haundavid