Audience Studio is introducing a solution that allows you to set a persistent ID when the user is on Firefox, with their default Enhanced Tracking Protection (ETP) policies. Now you have the option to implement client site managed 1p cookies or Audience Studio managed 1p cookies.
See Safari 1st Party Cookies
Changes to Google Same-Site Settings
for related content.
Site Managed 1p Cookie vs Audience Studio Managed 1p Cookies
Site administrators can avoid the problem of rapidly-expiring cookies by setting their web servers to enable a cookie automatically, instead of setting the cookie via the browser.
With this approach, when a user navigates to the client site, it issues an HTTP request for the page content. The server response includes a “Set-Cookie” HTTP header. Firefox reads this header and sets a cookie accordingly.
You can configure the Audience Studio Control Tag to read this 1p cookie and use its value as a device identifier. This technique, known as Site-Managed 1p Cookies, provides significantly higher accuracy counts due to the longer expiry times associated with these server-side configured cookies.