That statement from Avast does not appear to accord with the news article I just read.
The statement certainly implies that Mozilla have just made some general rule change, not specifically aimed at Avast, that happened to affect the Avast extension.
Mozilla have not made any rule change.
A researcher captured and studied all the user info being sent from the Avast and AVG security AND shopping extensions back to Avast, and found it was a tonne of data about machine details, software browsing history with IP addresses etc, a lot very personal and far more than required by either the Avast/AVG security or shopping extensions to carry out their functions. He reported this to Mozilla, Opera and Google, as well as publishing about it. Mozilla and Opera immediately responded to his reports by pulling the Avast and AVG extensions from their extension stores (while Google did nothing).
This was NOT Mozilla (and Opera) changing their extension store terms and that happening to catch out Avast/AVG along with others. This was one person finding that the Avast/AVG extensions have been harvesting vast amounts of personal data, way beyond what people would have expected, in clear violation of the terms the Mozilla and Opera extension stores have had for a long time. And those stores immediately removed the extensions when this was pointed out to them.
This is Avast being caught with its pants down, seriously violating user privacy. That Avast statement is just misleading spin trying to disguise the facts. It's certainly going to make me think twice about renewing.