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recieving java script error messages

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Every time I open Firefox, I get a JavaScript Application error message that says; Error: TypeError; CSSAndUICompatability_avg.ShowUIElementsByFilter is not a function

Every time I open Firefox, I get a JavaScript Application error message that says; Error: TypeError; CSSAndUICompatability_avg.ShowUIElementsByFilter is not a function

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That could be due to Malware. Check the Troubleshoot Firefox issues caused by malware article. You should probably download and run both Malwarebytes and SUPERAntiSpyware.