diff --git a/modules/libjar/nsJARChannel.cpp b/modules/libjar/nsJARChannel.cpp
index c3a17cc3e98..2148027b5bf 100644
--- a/modules/libjar/nsJARChannel.cpp
+++ b/modules/libjar/nsJARChannel.cpp
@@ -227,6 +227,22 @@ nsJARChannel::Init(nsIURI *uri)
 {
     nsresult rv;
     mJarURI = do_QueryInterface(uri, &rv);
+    if (NS_FAILED(rv))
+        return rv;
+
+    // Prevent loading jar:javascript URIs (see bug 290982).
+    nsCOMPtr<nsIURI> innerURI;
+    rv = mJarURI->GetJARFile(getter_AddRefs(innerURI));
+    if (NS_FAILED(rv))
+        return rv;
+    PRBool isJS;
+    rv = innerURI->SchemeIs("javascript", &isJS);
+    if (NS_FAILED(rv))
+        return rv;
+    if (isJS) {
+        NS_WARNING("blocking jar:javascript:");
+        return NS_ERROR_INVALID_ARG;
+    }
 
 #if defined(PR_LOGGING)
     mJarURI->GetSpec(mSpec);
diff --git a/netwerk/protocol/viewsource/src/nsViewSourceChannel.cpp b/netwerk/protocol/viewsource/src/nsViewSourceChannel.cpp
index 4409952e905..1d9237d5aa7 100644
--- a/netwerk/protocol/viewsource/src/nsViewSourceChannel.cpp
+++ b/netwerk/protocol/viewsource/src/nsViewSourceChannel.cpp
@@ -80,6 +80,17 @@ nsViewSourceChannel::Init(nsIURI* uri)
     nsCOMPtr<nsIIOService> pService(do_GetIOService(&rv));
     if (NS_FAILED(rv)) return rv;
 
+    nsCAutoString scheme;
+    rv = pService->ExtractScheme(path, scheme);
+    if (NS_FAILED(rv))
+      return rv;
+
+    // prevent viewing source of javascript URIs (see bug 204779)
+    if (scheme.LowerCaseEqualsLiteral("javascript")) {
+      NS_WARNING("blocking view-source:javascript:");
+      return NS_ERROR_INVALID_ARG;
+    }
+
     rv = pService->NewChannel(path, nsnull, nsnull, getter_AddRefs(mChannel));
     if (NS_FAILED(rv))
       return rv;