--- a/mail/themes/shared/mail/messageHeader.css
+++ b/mail/themes/shared/mail/messageHeader.css
@@ -182,16 +182,21 @@ mail-tagfield[collapsed="true"] {
 
 /* ::::: expanded header pane ::::: */
 
 #expandedsubjectBox {
   font-weight: bold;
   flex: 1;
   margin-inline-start: calc(var(--message-header-field-offset) * -1);
   padding-inline: var(--message-header-field-offset);
+  /* IMPORTANT! Keep these to avoid issues with very long subjects. Bug 77806 */
+  display: -webkit-box;
+  -webkit-box-orient: vertical;
+  -webkit-line-clamp: 3;
+  overflow-y: auto;
 }
 
 /* ::::: attachment view ::::: */
 
 #attachmentBar {
   color: -moz-dialogtext;
   background-color: -moz-Dialog;
   padding: 3px 0;