From: Ubuntu
Introduce a pref to prefer certain plugins for mime-types

For example:
 pref ("modules.plugins.mimetype.application/x-shockwave-flash", "/usr/lib/firefox-3.0.1/plugins/libflashplayer.so")

 would make the flashplyer installed in that location the preferred one to use.

diff --git a/dom/plugins/base/nsPluginHost.cpp b/dom/plugins/base/nsPluginHost.cpp
--- a/dom/plugins/base/nsPluginHost.cpp
+++ b/dom/plugins/base/nsPluginHost.cpp
@@ -1556,17 +1556,45 @@ nsPluginHost::FindPluginForType(const ch
                                 bool aCheckEnabled)
 {
   if (!aMimeType) {
     return nsnull;
   }
 
   LoadPlugins();
 
+  nsCAutoString mimetypePref("modules.plugins.mimetype.");
+  mimetypePref.Append(aMimeType);
+
+  nsAdoptingCString preferred = Preferences::GetCString(mimetypePref.get());
+
   nsPluginTag *plugin = mPlugins;
+  if(!preferred.IsEmpty()) {
+    nsCOMPtr<nsILocalFile> file;
+    nsresult rv = NS_NewNativeLocalFile(preferred, false, getter_AddRefs(file));
+    bool isAbsolute = NS_SUCCEEDED(rv);
+
+    while (plugin) {
+      if (((!isAbsolute && 0 == PL_strcasecmp(plugin->mFileName.get(), preferred.get())) ||
+            (isAbsolute && 0 == PL_strcasecmp(plugin->mFullPath.get(), preferred.get()))) &&
+          (!aCheckEnabled || plugin->IsEnabled())) {
+        PRInt32 mimeCount = plugin->mMimeTypes.Length();
+        for (PRInt32 i = 0; i < mimeCount; i++) {
+          if (0 == PL_strcasecmp(plugin->mMimeTypes[i].get(), aMimeType)) {
+            return plugin;
+          }
+        }
+      }
+      plugin = plugin->mNext;
+    }
+  }
+
+  // if there is no pref for this mime-type, or if the plugin named in pref
+  // isn't found, we pick the first that matches for this mime-type
+  plugin = mPlugins;
   while (plugin) {
     if (!aCheckEnabled || plugin->IsEnabled()) {
       PRInt32 mimeCount = plugin->mMimeTypes.Length();
       for (PRInt32 i = 0; i < mimeCount; i++) {
         if (0 == PL_strcasecmp(plugin->mMimeTypes[i].get(), aMimeType)) {
           return plugin;
         }
       }