diff --git a/mozilla/toolkit/mozapps/extensions/src/nsExtensionManager.js.in b/mozilla/toolkit/mozapps/extensions/src/nsExtensionManager.js.in
index 59c1f6bdb36..834e1538d68 100644
--- a/mozilla/toolkit/mozapps/extensions/src/nsExtensionManager.js.in
+++ b/mozilla/toolkit/mozapps/extensions/src/nsExtensionManager.js.in
@@ -8073,7 +8073,7 @@ ExtensionsDataSource.prototype = {
     if (id in this.visibleItems && this.visibleItems[id]) {
       var oldLocation = InstallLocations.get(this.visibleItems[id]);
       var newLocation = InstallLocations.get(locationKey);
-      if (forceReplace || newLocation.priority < oldLocation.priority)
+      if (forceReplace || !oldLocation || newLocation.priority < oldLocation.priority)
         this.visibleItems[id] = locationKey;
     }
     else
@@ -8177,6 +8177,8 @@ ExtensionsDataSource.prototype = {
   _getThemeImageURL: function(item, fileName, fallbackURL) {
     var id = stripPrefix(item.Value, PREFIX_ITEM_URI);
     var installLocation = this._em.getInstallLocation(id);
+    if (!installLocation)
+      return fallbackURL;
     var file = installLocation.getItemFile(id, fileName)
     if (file.exists())
       return gRDF.GetResource(getURLSpecFromFile(file));
diff --git a/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370.rdf b/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370.rdf
index d97c2e6280e..dfda3f3ad9a 100644
--- a/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370.rdf
+++ b/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370.rdf
@@ -42,9 +42,23 @@
       </RDF:Description>
     </em:targetApplication>
   </RDF:Description>
+  <RDF:Description RDF:about="urn:mozilla:item:bug356370_4@tests.mozilla.org">
+    <em:installLocation>invalid</em:installLocation>
+    <em:version>1</em:version>
+    <em:name>Bug 356370 test 4</em:name>
+    <em:type NC:parseType="Integer">4</em:type>
+    <em:targetApplication>
+      <RDF:Description>
+        <em:id>xpcshell@tests.mozilla.org</em:id>
+        <em:minVersion>1</em:minVersion>
+        <em:maxVersion>1</em:maxVersion>
+      </RDF:Description>
+    </em:targetApplication>
+  </RDF:Description>
   <RDF:Seq RDF:about="urn:mozilla:item:root">
     <RDF:li RDF:resource="urn:mozilla:item:bug356370_1@tests.mozilla.org"/>
     <RDF:li RDF:resource="urn:mozilla:item:bug356370_2@tests.mozilla.org"/>
     <RDF:li RDF:resource="urn:mozilla:item:bug356370_3@tests.mozilla.org"/>
+    <RDF:li RDF:resource="urn:mozilla:item:bug356370_4@tests.mozilla.org"/>
   </RDF:Seq>
 </RDF:RDF>
diff --git a/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370_4.rdf b/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370_4.rdf
new file mode 100644
index 00000000000..eb5d4d9a882
--- /dev/null
+++ b/mozilla/toolkit/mozapps/extensions/test/unit/data/test_bug356370_4.rdf
@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+
+<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+     xmlns:em="http://www.mozilla.org/2004/em-rdf#">
+
+  <Description about="urn:mozilla:install-manifest">
+    <em:id>bug356370_4@tests.mozilla.org</em:id>
+    <em:version>1</em:version>
+
+    <em:targetApplication>
+      <Description>
+        <em:id>xpcshell@tests.mozilla.org</em:id>
+        <em:minVersion>1</em:minVersion>
+        <em:maxVersion>1</em:maxVersion>
+      </Description>
+    </em:targetApplication>
+
+    <em:name>Bug 356370 test 4</em:name>
+    <em:type NC:parseType="Integer">4</em:type>
+  </Description>
+</RDF>
diff --git a/mozilla/toolkit/mozapps/extensions/test/unit/test_bug356370.js b/mozilla/toolkit/mozapps/extensions/test/unit/test_bug356370.js
index 5347e150178..b961fe22da2 100644
--- a/mozilla/toolkit/mozapps/extensions/test/unit/test_bug356370.js
+++ b/mozilla/toolkit/mozapps/extensions/test/unit/test_bug356370.js
@@ -44,10 +44,12 @@ function write_cache_line(stream, location, id, mtime) {
 /**
  * This copies two extensions, a default extensions datasource into the profile
  * It also manufactures an extensions.cache file with invalid items.
- * There are 3 test extensions:
+ * There are 4 test extensions:
  *   bug356370_1@tests.mozilla.org exists in app-profile and an unused version is in invalid-lo
- *   bug356370_2@tests.mozilla.org exists in invalid-hi and an unusd version is in app-profile
+ *   bug356370_2@tests.mozilla.org exists in invalid-hi and an unused version is in app-profile
  *   bug356370_3@tests.mozilla.org exists in invalid
+ *   bug356370_4@tests.mozilla.org is a theme existing in invalid and a new install
+ *     will be detected in app-profile
  *
  * After startup only the first two should exist in the correct install location
  * and installing extensions should be successful.
@@ -85,10 +87,18 @@ function setup_profile() {
   write_cache_line(foStream, "app-profile", "bug356370_2@tests.mozilla.org",
                    addon.lastModifiedTime);
 
+  addon = gProfD.clone();
+  addon.append("extensions");
+  addon.append("bug356370_4@tests.mozilla.org");
+  source = do_get_file("toolkit/mozapps/extensions/test/unit/data/test_bug356370_4.rdf");
+  addon.create(Components.interfaces.nsIFile.DIRECTORY_TYPE, 0755);
+  source.copyTo(addon, "install.rdf");
+
   // Write out a set of invalid entries
   write_cache_line(foStream, "invalid-lo", "bug356370_1@tests.mozilla.org", 0);
   write_cache_line(foStream, "invalid-hi", "bug356370_2@tests.mozilla.org", 0);
   write_cache_line(foStream, "invalid", "bug356370_3@tests.mozilla.org", 0);
+  write_cache_line(foStream, "invalid", "bug356370_4@tests.mozilla.org", 0);
   foStream.close();
 }
 
@@ -105,6 +115,8 @@ function run_test() {
   // This should still be disabled
   do_check_eq(getManifestProperty("bug356370_2@tests.mozilla.org", "isDisabled"), "true");
   do_check_eq(gEM.getItemForID("bug356370_3@tests.mozilla.org"), null);
+  do_check_neq(gEM.getItemForID("bug356370_4@tests.mozilla.org"), null);
+  do_check_eq(getManifestProperty("bug356370_4@tests.mozilla.org", "installLocation"), "app-profile");
 
   gEM.installItemFromFile(do_get_addon("test_bug257155"), NS_INSTALL_LOCATION_APPPROFILE);
   do_check_neq(gEM.getItemForID("bug257155@tests.mozilla.org"), null);
@@ -114,4 +126,5 @@ function run_test() {
   do_check_neq(gEM.getItemForID("bug356370_1@tests.mozilla.org"), null);
   do_check_neq(gEM.getItemForID("bug356370_2@tests.mozilla.org"), null);
   do_check_eq(gEM.getItemForID("bug356370_3@tests.mozilla.org"), null);
+  do_check_neq(gEM.getItemForID("bug356370_4@tests.mozilla.org"), null);
 }