User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

0. When Thunderbird launches, it usually shows a password dialog to authenticate with an IMAP server and receive new mail. In this case, Thunderbird has not been launched yet.
1. In Firefox, I click on a mailto link, which launches Thunderbird and opens a Write window. However, the password dialog to authenticate with an IMAP server does not appear.
2. When I click Send, Thunderbird tries to authenticate for sending.

Actual results:

Thunderbird shows a blank modal sheet and becomes unresponsive.

Expected results:

I believe when Thunderbird launches, the IMAP authentication dialog should appear first since it is modal. When clicking a mailto link, the Write window can be brought forward after dismissing the authentication dialog.

sounds like bug 530608, which is stuck on bug 476541