Cannot read data from local files with Firefox

I'm evaluating webstorm as a debugging environment for standalone JavaScript scripts that I use with a product called MaxMSP.
Whenever my code tries to load data from a file, I get the exception that's due to the default behavior of Firefox where it is not allowed to open local files.

However, I've found a number of places that describe how to change the behavior by adding some entries to about:config but no matter what I try, I continue to get the exception.
Can anyone point me at a definitive description of how to modify Firefox (latest version) to allow local files to be read?

Thanks,
David

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