Similar to a report on 102.0.2 (https://support.mozilla.org/en-US/questions/1382355), some messages are displayed as only a portion of HTML code, or the base-64 encoding of an attachment. Sometimes nothing is displayed. The header pane also displays HTML code rather than the message headers. It isn't the complete message source. First saw this yesterday on 91.13 and thought it might have stopped doing it later, but I may not have ever re-visited the offending messages. It happens with random messages from the same sender. I just installed 91.13.1 today with the same issue. (Lost my earlier edit, hope this isn't a re-post!)