PS: I am also attaching a screen shot from the about:config page with all "modified" items with the term "update" in them. To the best of my knowledge I have nothing modified here myself.

I'm not sure what causes that, but possibly the temporary folder where Firefox stages updates is locked or unable to be used. Hopefully someone who knows the file structure on Mac can point out where that is and you could empty it out and see whether that helps.

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