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gURLBar.handleCommand.toString().replace(/let/g, 'var'); // XXX eval fails when using let! Find why.

This is the problem. Some "let" should not be replaced by "var".
<script src="browserOverlay.js"/>

Add type="application/x-javascript" here to avoid eval failure.

Esta valoración es de una versión anterior del complemento ( 

Thanks for the tip ithinc! I appreciate it. :) Will test and update with the recommended change. Ajit.