--- a/dom/events/IMEContentObserver.cpp
+++ b/dom/events/IMEContentObserver.cpp
@@ -1679,24 +1679,29 @@ IMEContentObserver::IMENotificationSende
 
   if (observer->mNeedsToNotifyIMEOfTextChange) {
     observer->mNeedsToNotifyIMEOfTextChange = false;
     SendTextChange();
   }
 
   // If a text change notification causes another text change again, we should
   // notify IME of that before sending a selection change notification.
+  // Otherwise, even if the observer hasn't received selection change, let's
+  // try to send selection change notification to IME because selection
+  // start offset may be changed if the previous contents of selection start
+  // are changed.  For example, when previous `<p>` element of another `<p>`
+  // element which contains caret is removed by a DOM mutation, selection
+  // change event won't be fired, but selection start offset should be
+  // decreased by the length of removed `<p>` element.
   if (!observer->mNeedsToNotifyIMEOfTextChange) {
     // Be aware, PuppetWidget depends on the order of this. A selection change
     // notification should not be sent before a text change notification because
     // PuppetWidget shouldn't query new text content every selection change.
-    if (observer->mNeedsToNotifyIMEOfSelectionChange) {
-      observer->mNeedsToNotifyIMEOfSelectionChange = false;
-      SendSelectionChange();
-    }
+    observer->mNeedsToNotifyIMEOfSelectionChange = false;
+    SendSelectionChange();
   }
 
   // If a text change notification causes another text change again or a
   // selection change notification causes either a text change or another
   // selection change, we should notify IME of those before sending a position
   // change notification.
   if (!observer->mNeedsToNotifyIMEOfTextChange &&
       !observer->mNeedsToNotifyIMEOfSelectionChange) {

--- a/widget/tests/window_composition_text_querycontent.xul
+++ b/widget/tests/window_composition_text_querycontent.xul
@@ -8810,25 +8810,56 @@ async function runIMEContentObserverTest
     await waitNotifications;
     ensureToRemovePrecedingPositionChangeNotification();
     is(aElement.innerHTML, "<div>1<div>2<div>39</div></div></div>", description + " should remove '</div>4</div>5<div>6<div>7</div>8</div>'");
     // It causes removing '</div>4</div>5<div>6<div>7</div>8</div>' and inserting '<div>36<div>7</div>8</div>'.
     checkTextChangeNotification(notifications[0], description, { offset: getNativeText("\n1\n2\n3").length + offsetAtStart, removedLength: getNativeText("45\n6\n789").length, addedLength: getNativeText("9").length });
     checkSelectionChangeNotification(notifications[1], description, { offset: getNativeText("\n1\n2\n3").length + offsetAtStart, text: "" });
     checkPositionChangeNotification(notifications[2], description);
     dumpUnexpectedNotifications(description, 3);
+
+    // "<p>abc</p><p><br></p><p>{<br>}</p>" and removing second paragraph with DOM API
+    aElement.innerHTML = "<p>abc</p><p><br></p><p><br></p>";
+    sel.collapse(aElement.firstChild.nextSibling.nextSibling, 0);
+    await flushNotifications();
+    description = aDescription + "deleting previous paragraph with DOM API";
+    waitNotifications = promiseReceiveNotifications();
+    synthesizeKey("KEY_Unidentified", { code: "" }, win, callback);  // For setting the callback to recode notifications
+    aElement.firstChild.nextSibling.remove();
+    await waitNotifications;
+    ensureToRemovePrecedingPositionChangeNotification();
+    is(aElement.innerHTML, "<p>abc</p><p><br></p>", description + " the second paragraph should've been removed");
+    checkTextChangeNotification(notifications[0], description, { offset: getNativeText("\nabc").length + offsetAtStart, removedLength: getNativeText("\n\n").length, addedLength: 0 });
+    checkSelectionChangeNotification(notifications[1], description, { offset: getNativeText("\nabc\n").length + offsetAtStart, text: "" });
+    checkPositionChangeNotification(notifications[2], description);
+    dumpUnexpectedNotifications(description, 3);
+
+    // "<p>abc</p><p>{<br>}</p><p><br></p>" and removing last paragraph with DOM API
+    aElement.innerHTML = "<p>abc</p><p><br></p><p><br></p>";
+    sel.collapse(aElement.firstChild.nextSibling, 0);
+    await flushNotifications();
+    description = aDescription + "deleting next paragraph with DOM API";
+    waitNotifications = promiseReceiveNotifications();
+    synthesizeKey("KEY_Unidentified", { code: "" }, win, callback);  // For setting the callback to recode notifications
+    aElement.firstChild.nextSibling.nextSibling.remove();
+    await waitNotifications;
+    ensureToRemovePrecedingPositionChangeNotification();
+    is(aElement.innerHTML, "<p>abc</p><p><br></p>", description + " the last paragraph should've been removed");
+    checkTextChangeNotification(notifications[0], description, { offset: getNativeText("\nabc\n\n").length + offsetAtStart, removedLength: getNativeText("\n\n").length, addedLength: 0 });
+    checkPositionChangeNotification(notifications[1], description);
+    dumpUnexpectedNotifications(description, 2);
   }
 
   await testWithPlaintextEditor("runIMEContentObserverTest with input element: ", input, false);
   await testWithPlaintextEditor("runIMEContentObserverTest with textarea element: ", textarea, true);
   await testWithHTMLEditor("runIMEContentObserverTest with contenteditable (defaultParagraphSeparator is br): ", contenteditable, "br");
   await testWithHTMLEditor("runIMEContentObserverTest with contenteditable (defaultParagraphSeparator is p): ", contenteditable, "p");
   await testWithHTMLEditor("runIMEContentObserverTest with contenteditable (defaultParagraphSeparator is div): ", contenteditable, "div");
   // XXX Due to the difference of HTML editor behavior between designMode and contenteditable,
-  //     testWithHTMLEditor() gets some unexpected behavior.  However, IMEContentObserveri is
+  //     testWithHTMLEditor() gets some unexpected behavior.  However, IMEContentObserver is
   //     not depend on editor's detail.  So, we should investigate this issue later.  It's not
   //     so important for now.
   // await testWithHTMLEditor("runIMEContentObserverTest in designMode (defaultParagraphSeparator is br): ", iframe2.contentDocument.body, "br");
   // await testWithHTMLEditor("runIMEContentObserverTest in designMode (defaultParagraphSeparator is p): ", iframe2.contentDocument.body, "p");
   // await testWithHTMLEditor("runIMEContentObserverTest in designMode (defaultParagraphSeparator is div): ", iframe2.contentDocument.body, "div");
 }
 
 async function runPasswordMaskDelayTest() {