diff --git a/mail/components/extensions/parent/ext-compose.js b/mail/components/extensions/parent/ext-compose.js
index 30e4bfb84f..548696921c 100644
--- a/mail/components/extensions/parent/ext-compose.js
+++ b/mail/components/extensions/parent/ext-compose.js
@@ -675,6 +675,7 @@ async function setComposeDetails(composeWindow, details, extension) {
     )) {
       const recipientType = row.dataset.recipienttype.trim().toLowerCase();
       if (customHeaders.has(recipientType)) {
+        row.classList.remove("hidden");
         row.querySelector(".address-row-input").value =
           customHeaders.get(recipientType);
       }
diff --git a/mail/components/extensions/test/browser/browser_ext_compose_details_headers.js b/mail/components/extensions/test/browser/browser_ext_compose_details_headers.js
index f7124a6a01..079a2b5bc2 100644
--- a/mail/components/extensions/test/browser/browser_ext_compose_details_headers.js
+++ b/mail/components/extensions/test/browser/browser_ext_compose_details_headers.js
@@ -656,6 +656,11 @@ add_task(async function testCustomHeaders() {
       let customHeaders = [{ name: "X-TEST1", value: "some header" }];
       const tab = await browser.compose.beginNew(null, { customHeaders });
 
+      // The initial window should not have the X-Expediteur row visible.
+      await window.sendMessage("Check X-Expediteur header display", {
+        isHidden: true,
+      });
+
       // Add a header which does not start with X- and should not be touched by
       // the API.
       await window.sendMessage("addTestHeader");
@@ -687,6 +692,11 @@ add_task(async function testCustomHeaders() {
       ];
       await checkCustomHeaders(tab, expectedHeaders);
 
+      // The X-Expediteur row should now be visible.
+      await window.sendMessage("Check X-Expediteur header display", {
+        isHidden: false,
+      });
+
       // Update existing header and remove some of the others. Test support for
       // empty headers.
       customHeaders = [
@@ -764,6 +774,19 @@ add_task(async function testCustomHeaders() {
     extension.sendMessage(value);
   });
 
+  extension.onMessage("Check X-Expediteur header display", expected => {
+    const composeWindow = Services.wm.getMostRecentWindow("msgcompose");
+    const row = composeWindow.document.querySelector(
+      ".address-row-raw[data-recipienttype=X-Expediteur]"
+    );
+    Assert.equal(
+      BrowserTestUtils.isHidden(row),
+      expected.isHidden,
+      "The display of the X-Expediteur header row should be correct"
+    );
+    extension.sendMessage();
+  });
+
   await extension.startup();
   await extension.awaitFinish("finished");
   await extension.unload();