User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

1. Configure Thunderbird to show message body as plain text
2. Open a mail with attachments from Apple Mail

Actual results:

No attachments shown

Expected results:

Attachments shown

This is a bug in Apple Mail, but I doubt we'll see any fixes there so I'm hoping Thunderbird can work around their broken behaviour.

The problem is that Apple Mail organises the MIME structure so the attachments are with the HTML alternative, and not on the top level. Example mail (can't share all of it unfortunately):

Content-Type: multipart/alternative;
boundary="Apple-Mail=_561712E2-115F-45E0-9DEB-7E983247BD1C"
...
--Apple-Mail=_561712E2-115F-45E0-9DEB-7E983247BD1C
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain;
charset=us-ascii
...
--Apple-Mail=_561712E2-115F-45E0-9DEB-7E983247BD1C
Content-Type: multipart/mixed;
boundary="Apple-Mail=_41368DC5-35F9-420B-9CFA-3839D4D0C719"

--Apple-Mail=_41368DC5-35F9-420B-9CFA-3839D4D0C719
Content-Transfer-Encoding: 7bit
Content-Type: text/html;
charset=us-ascii
...
--Apple-Mail=_41368DC5-35F9-420B-9CFA-3839D4D0C719
Content-Disposition: attachment;
filename=tlclient.log
Content-Type: application/octet-stream;
x-unix-mode=0644;
name="tlclient.log"
Content-Transfer-Encoding: 7bit
...
--Apple-Mail=_41368DC5-35F9-420B-9CFA-3839D4D0C719--

--Apple-Mail=_561712E2-115F-45E0-9DEB-7E983247BD1C--

Claimed version of Apple Mail:

Mime-Version: 1.0 (Mac OS X Mail 14.0 (3654.0.3))
X-Mailer: Apple Mail (2.3654.0.3)