User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:71.0) Gecko/20100101 Firefox/71.0
Steps to reproduce:
I have previously stored a HTTP basic auth password for one site, let's call it sub1.example.org, in the Firefox password safe, which is protected with a master password.
After upgrading from Firefox 70 to 71, I am now prompted to enter my master password for a HTTP POST-based login form on site sub2.example.org, which has nothing to do with the basic auth login on the other subdomain. I control both subdomains so I know that their code hasn't changed (i.e. there is certainly no sub1.example.org frame embedded into sub2.example.org or similar).
If I do enter the master password, Firefox doesn't fill in any credentials on sub2.example.org, which is expected because the basic auth credentials from sub1.example.org do of course not match the HTTP POST input fields on sub2.example.org
I am asked to enter my master password although there are only passwords stored for an unrelated, different subdomain.
No password prompt, like in Firefox 70 and earlier.