If you search for Avast or AVG on the official Mozilla Add-ons website, you may notice that no results by these companies are returned. Neither Avast Online Security or SafePrice, nor AVG Online Security or SafePrice, are returned by the Store. The software is not blacklisted, so if you have any on your browser they will still work – for now.
The move Wladimir Palant, creator of AdBlock Plus, published an analysis of Avast extensions in late October 2019 on his personal site. He discovered that Avast's extension transmitted data to Avast that provided Avast with browsing history information. The data that the extension submits exceeded what is necessary to function.
The extensions include the full address of the page, the page title, referer, and other data in the request. Data is submitted when pages are opened but also when tabs are switched. On search pages, every single link on the page is submitted as well. Palant said:
The data collected here goes far beyond merely exposing the sites that you visit and your search history. Tracking tab and window identifiers as well as your actions allows Avast to create a nearly precise reconstruction of your browsing behavior: how many tabs do you have open, what websites do you visit and when, how much time do you spend reading/watching the contents, what do you click there and when do you switch to another tab. All that is connected to a number of attributes allowing Avast to recognize you reliably, even a unique user identifier.
Apparently Mozilla and Avast are locked in talks about modifying the add-ons. If the talks go badly then Avast could be blocked completely on Firefox.