diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index be516d479..d11002859 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -4855,6 +4855,12 @@ nsContentUtils::TriggerLink(nsIContent *aContent,
       fileName.SetIsVoid(true); // No actionable download attribute was found.
     }
 
+    // Sanitize fileNames containing null characters by replacing them with
+    // underscores.
+    if (!fileName.IsVoid()) {
+      fileName.ReplaceChar(char16_t(0), '_');
+    }
+
     nsDocShell::Cast(docShell)->OnLinkClick(aContent, aLinkURI,
                          fileName.IsVoid() ? aTargetSpec.get() : EmptyString().get(),
                          fileName, nullptr, nullptr, aIsTrusted);
diff --git a/dom/bindings/BindingUtils.cpp b/dom/bindings/BindingUtils.cpp
index 517dae97f..f0e223128 100644
--- a/dom/bindings/BindingUtils.cpp
+++ b/dom/bindings/BindingUtils.cpp
@@ -2290,6 +2290,12 @@ ConstructJSImplementation(JSContext* aCx, const char* aContractId,
   {
     AutoNoJSAPI nojsapi;
 
+    nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(aGlobal);
+    if (!window->IsCurrentInnerWindow()) {
+      aRv.Throw(NS_ERROR_FAILURE);
+      return;
+    }
+
     // Get the XPCOM component containing the JS implementation.
     nsresult rv;
     nsCOMPtr<nsISupports> implISupports = do_CreateInstance(aContractId, &rv);
@@ -2304,7 +2310,6 @@ ConstructJSImplementation(JSContext* aCx, const char* aContractId,
     // and our global is a window.
     nsCOMPtr<nsIDOMGlobalPropertyInitializer> gpi =
       do_QueryInterface(implISupports);
-    nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(aGlobal);
     if (gpi) {
       JS::Rooted<JS::Value> initReturn(aCx);
       rv = gpi->Init(window, &initReturn);
diff --git a/dom/html/nsTextEditorState.cpp b/dom/html/nsTextEditorState.cpp
index 008535ac4..3f4cf09bc 100644
--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -397,6 +397,7 @@ NS_IMETHODIMP
 nsTextInputSelectionImpl::SetCaretReadOnly(bool aReadOnly)
 {
   if (!mPresShellWeak) return NS_ERROR_NOT_INITIALIZED;
+  if (!mFrameSelection) return NS_ERROR_FAILURE;
   nsresult result;
   nsCOMPtr<nsIPresShell> shell = do_QueryReferent(mPresShellWeak, &result);
   if (shell)
diff --git a/netwerk/base/nsBaseChannel.cpp b/netwerk/base/nsBaseChannel.cpp
index e8ca7ccdc..dba5dafdd 100644
--- a/netwerk/base/nsBaseChannel.cpp
+++ b/netwerk/base/nsBaseChannel.cpp
@@ -575,6 +575,12 @@ NS_IMETHODIMP
 nsBaseChannel::SetContentDispositionFilename(const nsAString &aContentDispositionFilename)
 {
   mContentDispositionFilename = new nsString(aContentDispositionFilename);
+
+  // For safety reasons ensure the filename doesn't contain null characters and
+  // replace them with underscores. We may later pass the extension to system
+  // MIME APIs that expect null terminated strings.
+  mContentDispositionFilename->ReplaceChar(char16_t(0), '_');
+
   return NS_OK;
 }
 
diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index 89784819e..7ea31cb5b 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -509,6 +509,12 @@ NS_IMETHODIMP
 HttpBaseChannel::SetContentDispositionFilename(const nsAString& aContentDispositionFilename)
 {
   mContentDispositionFilename = new nsString(aContentDispositionFilename);
+
+  // For safety reasons ensure the filename doesn't contain null characters and
+  // replace them with underscores. We may later pass the extension to system
+  // MIME APIs that expect null terminated strings.
+  mContentDispositionFilename->ReplaceChar(char16_t(0), '_');
+
   return NS_OK;
 }
 
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index 18a14b87b..35ed2513f 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1288,6 +1288,8 @@ nsExternalAppHandler::nsExternalAppHandler(nsIMIMEInfo * aMIMEInfo,
 
   // replace platform specific path separator and illegal characters to avoid any confusion
   mSuggestedFileName.ReplaceChar(KNOWN_PATH_SEPARATORS FILE_ILLEGAL_CHARACTERS, '_');
+  // If null is in an extension, we should assert (see bug 1637745).
+  mSuggestedFileName.ReplaceChar(char16_t(0), '_');
   mTempFileExtension.ReplaceChar(KNOWN_PATH_SEPARATORS FILE_ILLEGAL_CHARACTERS, '_');
 
   // Remove unsafe bidi characters which might have spoofing implications (bug 511521).
@@ -2614,6 +2616,8 @@ NS_IMETHODIMP nsExternalHelperAppService::GetFromTypeAndExtension(const nsACStri
   NS_PRECONDITION(!aMIMEType.IsEmpty() ||
                   !aFileExt.IsEmpty(), 
                   "Give me something to work with");
+  MOZ_DIAGNOSTIC_ASSERT(aFileExt.FindChar('\0') == kNotFound,
+                        "The extension should never contain null characters");
   LOG(("Getting mimeinfo from type '%s' ext '%s'\n",
         PromiseFlatCString(aMIMEType).get(), PromiseFlatCString(aFileExt).get()));