Many times it really isn't Firefox or any other browser you might have installed. Most of the time they are some small unimportant file In your pc, mac, or other OS systems. As far as Windows, run a check on what file is missing in your browser. 9 out 10 times it's usually there where the problem is. For instance if you have a .dll file missing. Get a copy of it from microsoft and paste if on there. You'll see how simple it actually is. Our PC's or other OS update do much that it can throw a monkey wrench in our system without us really noticing until we use our favorite browser(firefox) is the case. Go back and check what is missing from the browser files, make a copy, and insert back into the browser.

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If you are having issues, please explain.