This is a bit odd: I have Malwarebytes set to run a threat scan every day at 17:00 EDT with update beforehand. Today it reported ARTUR.DUBOVOY@GMAIL.COM.XPI which is dated 3/20/2019 on my hard disk, and when I open it as a ZIP file everything is dated 3/18/2019 13:47. Nothing unusual in the manifest.json, but I didn't dig around further.
Thought it might be a false positive because it only showed up today, with the latest updates installed.
Scan Date: 5/8/19
Scan Time: 5:00 PM
Log File: 3e2cb0ea-71d4-11e9-b5a0-30b5c203ed65.json