NEVER EVER RUN FIREFOX AS ROOT!
To test if it starts and immediately close it down is OK
This is the behaviour in Windows and one of the reasons viruses spread so easily
But we have established that it is a permissions problem.
As you can use Opera no problems this is not urgent so I'll come back to you - I have not been able to do much in the forum a couple of days and the list of posts I have to go through and answer has grown long ..... but I'll come back