Hey everyone, I found the solution: Just add an exception for
https://fastmail.com
This took me way longer to figure out than it should have. Fastmail
switched over the URLS to include "app" in September/October 2022, but the cookies for whatever reason are still under the old URL. I figured this out by clicking on "manage data" and seeing the site the cookie was affiliated with.
For what it's worth, cookie management has changed on firefox recently, and those cookie settings do work slightly differently in very non-intuitive ways. You can check
this forum post for more details on how it currently works.
Hope this helps other people on here.
