User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

I composed an email to someone who I've emailed before and tried to send it.
I'm using ProtonMail Bridge (version 1.4.1) and I've never had this issue before (I've used ProtonMail Bridge for about two years).
I had just reinstalled Windows on my laptop and thus I had to install a fresh and up-to-date version of Thunderbird (78.3.1 32-bit).
I first installed Thunderbird 68 because I had a hard time downloading the up-to-date installer on my phone so as to avoid a slow hotspot.
I then updated Thunderbird to the most recent release and created my accounts, which are three ProtonMail accounts.
I used the config auto-detector to configure them each and they look right to me. I can receive emails on all of them just fine.
I've also reached out to ProtonMail in case the issue is on their end. Right now this issue makes me unable to use Thunderbird to send emails. Instead I'm using ProtonMail's web interface and the ProtonMail app.

Actual results:

When I hit "send", I got a popup (attached) that said:
"Sending of the message failed.
The certificate is not trusted because it is self-signed.
The configuration related to 127.0.0.1 must be corrected."
Thunderbird then failed to send the email without giving me the option to override or to trust this certificate.

Expected results:

Thunderbird should have sent the email successfully without incident. As far as I can tell my config is correct. At very least it receives emails just fine.
I would have preferred, though, that Thunderbird gave me the option of trusting the self-signed certificate, since it belongs to a trusted program running on the same machine as Thunderbird.

In case it's relevant:
After I created the accounts in Thunderbird, I changed the mail folder directory to an exact copy of their corresponding mail folders from before I reinstalled Windows and Thunderbird. Before the reinstall, the folders were from either the current version of Thunderbird or the previous release, so it probably isn't the cause of the problem, but I'm mentioning it in case it's relevant.

Also, I'm using maildir for all three mail folders.