Browser extensions were main data source for their analytics subsidiary Jumpshot [1], which was shutdown for privacy reasons. Before that, when Mozilla and Opera started asking about their extensions, they rather removed them [2].
[1]: https://www.vice.com/en/article/qjdkq7/avast-antivirus-sells...
[2]: https://palant.info/2019/12/03/mozilla-removes-avast-extensi...