--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -1297,16 +1297,19 @@ PresShell::Destroy()
   }
 
   if (mAccessibleCaretEventHub) {
     mAccessibleCaretEventHub->Terminate();
     mAccessibleCaretEventHub = nullptr;
   }
 
   // release our pref style sheet, if we have one still
+  //
+  // FIXME(emilio): Why do we need to do this? The stylist is getting nixed with
+  // us anyway.
   RemovePreferenceStyles();
 
   mIsDestroying = true;
 
   // We can't release all the event content in
   // mCurrentEventContentStack here since there might be code on the
   // stack that will release the event content too. Double release
   // bad!
@@ -1487,27 +1490,31 @@ PresShell::UpdatePreferenceStyles()
   if (mPrefStyleSheet == newPrefSheet) {
     return;
   }
 
   mStyleSet->BeginUpdate();
 
   RemovePreferenceStyles();
 
-  mStyleSet->AppendStyleSheet(SheetType::User, newPrefSheet);
+  // NOTE(emilio): This sheet is added as an agent sheet, because we don't want
+  // it to be modifiable from devtools and similar, see bugs 1239336 and
+  // 1436782. I think it conceptually should be a user sheet, and could be
+  // without too much trouble I'd think.
+  mStyleSet->AppendStyleSheet(SheetType::Agent, newPrefSheet);
   mPrefStyleSheet = newPrefSheet;
 
   mStyleSet->EndUpdate();
 }
 
 void
 PresShell::RemovePreferenceStyles()
 {
   if (mPrefStyleSheet) {
-    mStyleSet->RemoveStyleSheet(SheetType::User, mPrefStyleSheet);
+    mStyleSet->RemoveStyleSheet(SheetType::Agent, mPrefStyleSheet);
     mPrefStyleSheet = nullptr;
   }
 }
 
 void
 PresShell::AddUserSheet(StyleSheet* aSheet)
 {
   // Make sure this does what nsDocumentViewer::CreateStyleSet does wrt