I almost exclusively access Gmail with Mozilla Firefox on my laptop running Microsoft Windows 10. The Avast footer was added to emails I sent through this Gmail web interface.

Not only that, but the Avast footer isn’t even implemented properly: I have the format of my outgoing emails set to plain text, but Avast adds the footer encoded in HTML, so the recipients of my emails just get a load of HTML garbage at the end of the email. I wonder what they think I’m up to!