Sometimes I receive a message that is encoded as: Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable This is not displayed properly. If I use Gmail in Firefox, it is displayed correctly. If I download the message part, it displays properly in Firefox. To me, this implies a problem in the decoding engine of Thunderbird. FWIW, I have tried running in safe mode but I get the same results. I am running on Thunderbird 52.9.1 because I am using add-ons that have not yet been upgraded. BTW, I did Google for this error and found some closed tickets from 9 - 10 years ago that sound similar.