Linux Mint Forums

That's odd
Luckily Linux has a simple but efficient built in debug tool - the terminal
Use the terminal to start Firefox (the command is simply firefox )
Then look for error messages in the terminal - you may find the answer there or at least you can post it here

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

You know computers are a bit like living organisms - you never really know what will happen.
This is because there is an element of randomness - just as this or that important file is written there's a power spike (or brown out) and it does not go well.....