diff --git a/extensions/spellcheck/src/mozInlineSpellChecker.cpp b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
index 3df2ba511952..c3e00a3995b3 100644
--- a/extensions/spellcheck/src/mozInlineSpellChecker.cpp
+++ b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
@@ -1058,12 +1058,18 @@ bool mozInlineSpellChecker::ShouldSpellCheckNode(TextEditor* aTextEditor,
                                            nsGkAtoms::cite, eIgnoreCase)) {
         return false;
       }
-      if (parent->IsHTMLElement(nsGkAtoms::pre) &&
+      if (parent->IsAnyOfHTMLElements(nsGkAtoms::pre, nsGkAtoms::div) &&
           parent->AsElement()->AttrValueIs(kNameSpaceID_None, nsGkAtoms::_class,
                                            nsGkAtoms::mozsignature,
                                            eIgnoreCase)) {
         return false;
       }
+      if (parent->IsHTMLElement(nsGkAtoms::div) &&
+          parent->AsElement()->AttrValueIs(kNameSpaceID_None, nsGkAtoms::_class,
+                                           nsGkAtoms::mozfwcontainer,
+                                           eIgnoreCase)) {
+        return false;
+      }
 
       parent = parent->GetParent();
     }
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index 1b95b2c4e9a3..c38cae9a8475 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -42,12 +42,13 @@ STATIC_ATOMS = [
     Atom("mozdirty", "_moz_dirty"),
     Atom("mozdisallowselectionprint", "mozdisallowselectionprint"),
     Atom("mozdonotsend", "moz-do-not-send"),
+    Atom("mozfwcontainer", "moz-forward-container"),  # Used by MailNews.
     Atom("mozgeneratedcontentbefore", "_moz_generated_content_before"),
     Atom("mozgeneratedcontentafter", "_moz_generated_content_after"),
     Atom("mozgeneratedcontentmarker", "_moz_generated_content_marker"),
     Atom("mozgeneratedcontentimage", "_moz_generated_content_image"),
     Atom("mozquote", "_moz_quote"),
-    Atom("mozsignature", "moz-signature"),
+    Atom("mozsignature", "moz-signature"),  # Used by MailNews.
     Atom("_moz_is_glyph", "-moz-is-glyph"),
     Atom("_moz_original_size", "_moz_original_size"),
     Atom("menuactive", "_moz-menuactive"),