--- a/browser/components/search/test/browser_webapi.js
+++ b/browser/components/search/test/browser_webapi.js
@@ -3,21 +3,17 @@ const searchBundle = Services.strings.cr
 const brandBundle = Services.strings.createBundle("chrome://branding/locale/brand.properties");
 const brandName = brandBundle.GetStringFromName("brandShortName");
 
 function getString(key, ...params) {
   return searchBundle.formatStringFromName(key, params, params.length);
 }
 
 function AddSearchProvider(...args) {
-  return gBrowser.addTab(ROOT + "webapi.html?AddSearchProvider:" + encodeURIComponent(JSON.stringify(args)));
-}
-
-function addSearchEngine(...args) {
-  return gBrowser.addTab(ROOT + "webapi.html?addSearchEngine:" + encodeURIComponent(JSON.stringify(args)));
+  return gBrowser.addTab(ROOT + "webapi.html?" + encodeURIComponent(JSON.stringify(args)));
 }
 
 function promiseDialogOpened() {
   return new Promise((resolve, reject) => {
     Services.wm.addListener({
       onOpenWindow: function(xulWin) {
         Services.wm.removeListener(this);
 
@@ -29,153 +25,68 @@ function promiseDialogOpened() {
           else
             reject();
         }, win);
       }
     });
   });
 }
 
-add_task(function* test_working_AddSearchProvider() {
+add_task(function* test_working() {
   gBrowser.selectedTab = AddSearchProvider(ROOT + "testEngine.xml");
 
   let dialog = yield promiseDialogOpened();
   is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
   is(dialog.args.text, getString("addEngineConfirmation", "Foo", "example.com"),
      "Should have seen the right install message");
   dialog.document.documentElement.cancelDialog();
 
   gBrowser.removeCurrentTab();
 });
 
-add_task(function* test_HTTP_AddSearchProvider() {
+add_task(function* test_HTTP() {
   gBrowser.selectedTab = AddSearchProvider(ROOT.replace("http:", "HTTP:") + "testEngine.xml");
 
   let dialog = yield promiseDialogOpened();
   is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
   is(dialog.args.text, getString("addEngineConfirmation", "Foo", "example.com"),
      "Should have seen the right install message");
   dialog.document.documentElement.cancelDialog();
 
   gBrowser.removeCurrentTab();
 });
 
-add_task(function* test_relative_AddSearchProvider() {
+add_task(function* test_relative() {
   gBrowser.selectedTab = AddSearchProvider("testEngine.xml");
 
   let dialog = yield promiseDialogOpened();
   is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
   is(dialog.args.text, getString("addEngineConfirmation", "Foo", "example.com"),
      "Should have seen the right install message");
   dialog.document.documentElement.cancelDialog();
 
   gBrowser.removeCurrentTab();
 });
 
-add_task(function* test_invalid_AddSearchProvider() {
+add_task(function* test_invalid() {
   gBrowser.selectedTab = AddSearchProvider("z://foobar");
 
   let dialog = yield promiseDialogOpened();
   is(dialog.args.promptType, "alert", "Should see the alert dialog.");
   is(dialog.args.text, getString("error_invalid_engine_msg", brandName),
      "Should have seen the right error message")
   dialog.document.documentElement.acceptDialog();
 
   gBrowser.removeCurrentTab();
 });
 
-add_task(function* test_missing_AddSearchProvider() {
+add_task(function* test_missing() {
   let url = ROOT + "foobar.xml";
   gBrowser.selectedTab = AddSearchProvider(url);
 
   let dialog = yield promiseDialogOpened();
   is(dialog.args.promptType, "alert", "Should see the alert dialog.");
   is(dialog.args.text, getString("error_loading_engine_msg2", brandName, url),
      "Should have seen the right error message")
   dialog.document.documentElement.acceptDialog();
 
   gBrowser.removeCurrentTab();
 });
-
-add_task(function* test_working_addSearchEngine_xml() {
-  gBrowser.selectedTab = addSearchEngine(ROOT + "testEngine.xml", "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
-  is(dialog.args.text, getString("addEngineConfirmation", "Foo", "example.com"),
-     "Should have seen the right install message");
-  dialog.document.documentElement.cancelDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_working_addSearchEngine_src() {
-  gBrowser.selectedTab = addSearchEngine(ROOT + "testEngine.src", "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
-  is(dialog.args.text, getString("addEngineConfirmation", "Test Sherlock", "example.com"),
-     "Should have seen the right install message");
-  dialog.document.documentElement.cancelDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_relative_addSearchEngine_xml() {
-  gBrowser.selectedTab = addSearchEngine("testEngine.xml", "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
-  is(dialog.args.text, getString("addEngineConfirmation", "Foo", "example.com"),
-     "Should have seen the right install message");
-  dialog.document.documentElement.cancelDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_relative_addSearchEngine_src() {
-  gBrowser.selectedTab = addSearchEngine("testEngine.src", "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "confirmEx", "Should see the confirmation dialog.");
-  is(dialog.args.text, getString("addEngineConfirmation", "Test Sherlock", "example.com"),
-     "Should have seen the right install message");
-  dialog.document.documentElement.cancelDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_invalid_addSearchEngine() {
-  gBrowser.selectedTab = addSearchEngine("z://foobar", "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "alert", "Should see the alert dialog.");
-  is(dialog.args.text, getString("error_invalid_engine_msg", brandName),
-     "Should have seen the right error message")
-  dialog.document.documentElement.acceptDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_invalid_icon_addSearchEngine() {
-  gBrowser.selectedTab = addSearchEngine(ROOT + "testEngine.src", "z://foobar", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "alert", "Should see the alert dialog.");
-  is(dialog.args.text, getString("error_invalid_engine_msg", brandName),
-     "Should have seen the right error message")
-  dialog.document.documentElement.acceptDialog();
-
-  gBrowser.removeCurrentTab();
-});
-
-add_task(function* test_missing_addSearchEngine() {
-  let url = ROOT + "foobar.xml";
-  gBrowser.selectedTab = addSearchEngine(url, "", "", "");
-
-  let dialog = yield promiseDialogOpened();
-  is(dialog.args.promptType, "alert", "Should see the alert dialog.");
-  is(dialog.args.text, getString("error_loading_engine_msg2", brandName, url),
-     "Should have seen the right error message")
-  dialog.document.documentElement.acceptDialog();
-
-  gBrowser.removeCurrentTab();
-});

--- a/browser/components/search/test/webapi.html
+++ b/browser/components/search/test/webapi.html
@@ -1,20 +1,16 @@
 <!DOCTYPE html>
 
 <html>
 <head>
 <script>
 function installEngine() {
-  var query = window.location.search.substring(1).split(":");
-  var func = query[0];
-  var args = JSON.parse(decodeURIComponent(query[1]));
+  var query = window.location.search.substring(1);
+  var args = JSON.parse(decodeURIComponent(query));
 
-  if (func == "AddSearchProvider")
-    window.external.AddSearchProvider(...args);
-  else if (func == "addSearchEngine")
-    window.sidebar.addSearchEngine(...args);
+  window.external.AddSearchProvider(...args);
 }
 </script>
 </head>
 <body onload="installEngine()">
 </body>
 </html>

--- a/toolkit/components/processsingleton/MainProcessSingleton.js
+++ b/toolkit/components/processsingleton/MainProcessSingleton.js
@@ -19,43 +19,38 @@ MainProcessSingleton.prototype = {
                                          Ci.nsISupportsWeakReference]),
 
   logConsoleMessage: function(message) {
     let logMsg = message.data;
     logMsg.wrappedJSObject = logMsg;
     Services.obs.notifyObservers(logMsg, "console-api-log-event", null);
   },
 
-  // Called when a webpage calls either window.external.AddSearchProvider or
-  // window.sidebar.addSearchEngine
-  addSearchEngine: function({ target: browser, data: { pageURL, engineURL, iconURL, type } }) {
+  // Called when a webpage calls window.external.AddSearchProvider
+  addSearchEngine: function({ target: browser, data: { pageURL, engineURL, type } }) {
     pageURL = NetUtil.newURI(pageURL);
     engineURL = NetUtil.newURI(engineURL, null, pageURL);
 
-    if (iconURL) {
-      iconURL = NetUtil.newURI(iconURL, null, pageURL);
-    }
-    else {
-      let tabbrowser = browser.getTabBrowser();
-      if (browser.mIconURL && (!tabbrowser || tabbrowser.shouldLoadFavIcon(pageURL)))
-        iconURL = NetUtil.newURI(browser.mIconURL);
-    }
+    let iconURL;
+    let tabbrowser = browser.getTabBrowser();
+    if (browser.mIconURL && (!tabbrowser || tabbrowser.shouldLoadFavIcon(pageURL)))
+      iconURL = NetUtil.newURI(browser.mIconURL);
 
     try {
       // Make sure the URLs are HTTP, HTTPS, or FTP.
       let isWeb = ["https", "http", "ftp"];
 
       if (isWeb.indexOf(engineURL.scheme) < 0)
         throw "Unsupported search engine URL: " + engineURL;
 
       if (iconURL && isWeb.indexOf(iconURL.scheme) < 0)
         throw "Unsupported search icon URL: " + iconURL;
     }
     catch(ex) {
-      Cu.reportError("Invalid argument passed to window.sidebar.addSearchEngine: " + ex);
+      Cu.reportError("Invalid argument passed to window.external.AddSearchProvider: " + ex);
 
       var searchBundle = Services.strings.createBundle("chrome://global/locale/search/search.properties");
       var brandBundle = Services.strings.createBundle("chrome://branding/locale/brand.properties");
       var brandName = brandBundle.GetStringFromName("brandShortName");
       var title = searchBundle.GetStringFromName("error_invalid_engine_title");
       var msg = searchBundle.formatStringFromName("error_invalid_engine_msg",
                                                   [brandName], 1);
       Services.ww.getNewPrompter(browser.ownerDocument.defaultView).alert(title, msg);

--- a/toolkit/components/search/nsSidebar.js
+++ b/toolkit/components/search/nsSidebar.js
@@ -17,34 +17,29 @@ nsSidebar.prototype = {
   init: function(window) {
     this.window = window;
     this.mm = window.QueryInterface(Ci.nsIInterfaceRequestor)
                     .getInterface(Ci.nsIDocShell)
                     .QueryInterface(Ci.nsIInterfaceRequestor)
                     .getInterface(Ci.nsIContentFrameMessageManager);
   },
 
-  // The suggestedTitle and suggestedCategory parameters are ignored, but remain
-  // for backward compatibility.
+  // Deprecated, only left here to avoid breaking old browser-detection scripts.
   addSearchEngine: function(engineURL, iconURL, suggestedTitle, suggestedCategory) {
-    let dataType = SHERLOCK_FILE_EXT_REGEXP.test(engineURL) ?
-                   Ci.nsISearchEngine.DATA_TEXT :
-                   Ci.nsISearchEngine.DATA_XML;
+    if (SHERLOCK_FILE_EXT_REGEXP.test(engineURL)) {
+      Cu.reportError("Installing Sherlock search plugins is no longer supported.");
+      return;
+    }
 
-    this.mm.sendAsyncMessage("Search:AddEngine", {
-      pageURL: this.window.document.documentURIObject.spec,
-      engineURL,
-      type: dataType,
-      iconURL
-    });
+    this.AddSearchProvider(engineURL);
   },
 
-  // This function exists largely to implement window.external.AddSearchProvider(),
-  // to match other browsers' APIs.  The capitalization, although nonstandard here,
-  // is therefore important.
+  // This function implements window.external.AddSearchProvider().
+  // The capitalization, although nonstandard here, is to match other browsers'
+  // APIs and is therefore important.
   AddSearchProvider: function(engineURL) {
     this.mm.sendAsyncMessage("Search:AddEngine", {
       pageURL: this.window.document.documentURIObject.spec,
       engineURL,
       type: Ci.nsISearchEngine.DATA_XML
     });
   },