I found and disabled this feature. PLEASE DO NOT DO THIS. I don't want the people I sent emails to seeing some crappy Avast ad. It's also super creepy that it can invade my browser even with the Firefox plugin disabled. (Which it enabled without my permission AGAIN. I complained about this exact same thing earlier.)
This actually has nothing to do with the issue you reported previously, it isn't a firefox add-on.
You don't say what version of avast that you have installed, so I will have to presume it is the latest 2016.11.1.2253:
I suspect you have switched this off in the Mail Shield, but that doesn't work for webmail and why it has reverted.
Check the AvastUI > Settings > General - uncheck the 'Enable Avast email signature.'