If you search for Avast or AVG on the official Mozilla add-on website, you may notice that these companies have not returned any results. Mozilla appears to have removed these extensions from its store. Mozilla did not blacklist the Avast Firefox extension. Blacklisted extensions are placed in the block list and are therefore removed from the user’s browser.
Avast and AVG extensions have been removed, but are not blocked, which means that they remain in the Firefox browser for the time being. Mozilla added dozens of Firefox extensions to the block list on December 2, 2019, collecting user data without disclosure or consent, but Avast’s extensions are not on the list.
Wladimir Palant, founder of AdBlock Plus, posted an analysis of the Avast extension on its personal website in late October 2018. He discovered that Avast’s extension transferred data to Avast, providing Avast with browsing history. The extended submission of data exceeds the data required to run.
This data allows Avast to perform almost precise rebuildings of a user’s browsing behavior, including how many tabs were opened, which websites were accessed, and when and how much time it took to read/watch content, allowing Avast to reliably identify users, even unique user identifiers.