Doh, found the root cause, let this be a cautionary tale for all of us. Apparently (?) Firefox add-on stores the signing date to its signature, and when add-on is installed to Firefox, a signature date seeming to be in future is rejected as corrupted. In other words, I was testing with VMware image and my snapshot’s date was not correct, i.e. the image’s clock was showing a date older than add-on signing date. And that caused add-on the be flagged as corrupted. As soon as I manually adjust the image’s clock, add-on installation started working. 
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