diff --git a/editor/libeditor/HTMLEditor.cpp b/editor/libeditor/HTMLEditor.cpp
index b10d4abfa7cc..452a162d4f5f 100644
--- a/editor/libeditor/HTMLEditor.cpp
+++ b/editor/libeditor/HTMLEditor.cpp
@@ -4699,6 +4699,22 @@ nsresult HTMLEditor::EnsureNoFollowingUnnecessaryLineBreak(
     return NS_OK;
   }
   if (unnecessaryLineBreak->IsHTMLBRElement()) {
+    // If the found unnecessary <br> is a preceding one of a mailcite which is a
+    // <span> styled as block, we need to preserve the <br> element for the
+    // serializer to cause a line break before the mailcite.
+    if (IsPlaintextMailComposer()) {
+      const WSScanResult nextThing =
+          WSRunScanner::ScanInclusiveNextVisibleNodeOrBlockBoundary(
+              WSRunScanner::Scan::All,
+              unnecessaryLineBreak->After<EditorRawDOMPoint>(),
+              BlockInlineCheck::UseComputedDisplayOutsideStyle);
+      if (nextThing.ReachedOtherBlockElement() &&
+          HTMLEditUtils::IsMailCite(*nextThing.ElementPtr()) &&
+          HTMLEditUtils::IsInlineContent(
+              *nextThing.ElementPtr(), BlockInlineCheck::UseHTMLDefaultStyle)) {
+        return NS_OK;
+      }
+    }
     // If the invisible break is a placeholder of ancestor inline elements, we
     // should not delete it to allow users to insert text with the format
     // specified by them.
diff --git a/editor/libeditor/tests/mochitest.toml b/editor/libeditor/tests/mochitest.toml
index 25597ff5c258..a1f1385d475f 100644
--- a/editor/libeditor/tests/mochitest.toml
+++ b/editor/libeditor/tests/mochitest.toml
@@ -479,6 +479,9 @@ skip-if = ["display == 'wayland'"] # Bug 1935188
 ["test_mailcite_backspace_at_end_of_inline_reply.html"]
 skip-if = ["xorigin"] # Testing internal API for comm-central
 
+["test_mailcite_keep_preceding_br_after_insert.html"]
+skip-if = ["xorigin"] # Testing internal API for comm-central
+
 ["test_mailcite_keep_trailing_br_after_delete.html"]
 skip-if = ["xorigin"] # Testing internal API for comm-central
 
diff --git a/editor/libeditor/tests/test_mailcite_keep_preceding_br_after_insert.html b/editor/libeditor/tests/test_mailcite_keep_preceding_br_after_insert.html
new file mode 100644
index 000000000000..6169b16d793c
--- /dev/null
+++ b/editor/libeditor/tests/test_mailcite_keep_preceding_br_after_insert.html
@@ -0,0 +1,73 @@
+<!doctype html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>Preceding &lt;br&gt; of a mailcite which is a blocked &lt;span&gt;</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<link rel="stylesheet" href="/tests/SimpleTest/test.css">
+<script>
+"use strict";
+
+SimpleTest.waitForExplicitFinish();
+SimpleTest.waitForFocus(async () => {
+  const iframe = document.querySelector("iframe");
+  await new Promise(resolve => {
+    if (iframe.contentDocument?.readyState == "complete") {
+      resolve();
+      return;
+    }
+    iframe.addEventListener("load", resolve, {once: true});
+  });
+
+
+  const win = iframe.contentWindow;
+  getEditor(win).flags |=
+    SpecialPowers.Ci.nsIEditor.eEditorMailMask | SpecialPowers.Ci.nsIEditor.eEditorPlaintextMask;
+  const doc = iframe.contentDocument;
+  const mailEditor = getEditorMailSupport(win);
+  win.focus();
+  doc.body.focus();
+  const mailciteStyle = "white-space: pre-wrap; display: block; width: 98vw;";
+  const mailcite = SpecialPowers.unwrap(
+    mailEditor.insertAsCitedQuotation("This is a mail cite", "", false)
+  );
+  is(
+    doc.body.innerHTML,
+    `<span style="${mailciteStyle}">&gt; This is a mail cite<br><br></span><br><br>`,
+    "nsIEditorMailSupport.insertAsCitedQuotation() should insert a mailcite span"
+  );
+  // Split paragraph between "a " and "mail cite".
+  win.getSelection().collapse(mailcite.firstChild, "> This is a ".length);
+  doc.execCommand("insertParagraph");
+  is(
+    doc.body.innerHTML,
+    `<span style="${mailciteStyle}">&gt; This is a <br></span><br><span style="${mailciteStyle}">mail cite<br><br></span><br><br>`,
+    "insertParagraph in a mailcite should split the mailcite and insert an empty line"
+  );
+  // Then, type a character. The <br> before the latter mailcite should be
+  // preserved for the serializer to insert a line break before the latter
+  // mailcite.
+  doc.execCommand("insertText", false, "X");
+  is(
+    doc.body.innerHTML,
+    `<span style="${mailciteStyle}">&gt; This is a <br></span>X<br><span style="${mailciteStyle}">mail cite<br><br></span><br><br>`,
+    "Typing text into the empty line should preserve the preceding <br> of the latter mailcite"
+  );
+
+  SimpleTest.finish();
+});
+
+function getEditor(aWindow) {
+  const editingSession = SpecialPowers.wrap(aWindow).docShell.editingSession;
+  return editingSession.getEditorForWindow(aWindow);
+}
+
+function getEditorMailSupport(aWindow) {
+  return getEditor(aWindow).QueryInterface(SpecialPowers.Ci.nsIEditorMailSupport);
+}
+</script>
+</head>
+<body>
+  <iframe srcdoc="<body contenteditable><br><br></body>"></iframe>
+</body>
+</html>
