The other day I send an email through Gmail in Firefox. Right after I clicked "Send" Avast popped up asking if I wanted to include their signature at the bottom of future emails. Surprised, I accepted and proceeded to send another email. I was shocked to see that it worked, it actually added their message "This email is virus-free".
Now, my question is, how is that possible? How can third-party software change email content I'm sending though Gmail with https encryption from Firefox?