Can you open Adobe Reader by itself? If so, try to disable Protected Mode [Edit | Preferences | Security (Enhanced)].
P.S. Firefox uses its own PDF viewer, not the Adobe Reader plugin.
P.P.S. if you mark and answer as 'Correct' then the topic gets marked 'Resolved', and it is very likely that no-one looks at it again. It is by purest chance that I looked at your resolved topic.