Thunderbird allowed feed subscriptions to file: URLs that point to a network share. If a user was tricked into subscribing to such a URL, Thunderbird connected to the attacker's server when fetching the feed. Windows then sent the user's NTLM credentials to that server.
This bug only affects Thunderbird on Windows. Other operating systems are unaffected. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.