The No More 404s experiment, like all of Test Pilot, is totally opt-in. The whole idea is to let us try things with Firefox, get feedback and iterate quickly.

No More 404s Telemetry ping is only gathering data about how often the Add-on is fired and how often it is clicked. We (Mozilla) don't know anything about URLs. The Wayback Machine has it's own privacy protections in place that you can learn about here: https://blog.archive.org/2013/10/25/reader-privacy-at-the-in...

Also worth noting, every Test Pilot experiment comes with a brief explainer of all data collections. Here's the explainer for No More 404s:

In addition to the data collected by all Test Pilot experiments, here are the key things you should know about what is happening when you use No More 404s:

* We collect basic usage on how many times you encounter a Page Not Found error (code 404), how many times a cached version of that page exists from Archive.org, and how many times you choose to view the cached version.

* To provide cached versions of pages, we send 404 error page URLs to Archive.org. Archive.org discloses its privacy policy here (https://archive.org/about/terms.php).

* We do not collect URLs of the pages you request or the URLs we send to Archive.org.

* We may share survey results you submit to us and aggregated telemetry data related to this experiment with the Internet Archive.