--- a/toolkit/components/autocomplete/nsAutoCompleteController.cpp
+++ b/toolkit/components/autocomplete/nsAutoCompleteController.cpp
@@ -495,26 +495,37 @@ nsAutoCompleteController::HandleKeyNavig
         if (NS_SUCCEEDED(GetResultValueAt(selectedIndex, false, value))) {
           input->SetTextValue(value);
           input->SelectTextRange(value.Length(), value.Length());
         }
       }
       else if (shouldComplete) {
         // We usually try to preserve the casing of what user has typed, but
         // if he wants to autocomplete, we will replace the value with the
-        // actual autocomplete result.
+        // actual autocomplete result. Note that the autocomplete input can also
+        // be showing e.g. "bar >> foo bar" if the search matched "bar", a
+        // word not at the start of the full value "foo bar".
         // The user wants explicitely to use that result, so this ensures
         // association of the result with the autocompleted text.
         nsAutoString value;
         nsAutoString inputValue;
         input->GetTextValue(inputValue);
-        if (NS_SUCCEEDED(GetDefaultCompleteValue(-1, false, value)) &&
-            value.Equals(inputValue, nsCaseInsensitiveStringComparator())) {
-          input->SetTextValue(value);
-          input->SelectTextRange(value.Length(), value.Length());
+        if (NS_SUCCEEDED(GetDefaultCompleteValue(-1, false, value))) {
+          nsAutoString suggestedValue;
+          int32_t pos = inputValue.Find(" >> ");
+          if (pos > 0) {
+            inputValue.Right(suggestedValue, inputValue.Length() - pos - 4);
+          } else {
+            suggestedValue = inputValue;
+          }
+
+          if (value.Equals(suggestedValue, nsCaseInsensitiveStringComparator())) {
+            input->SetTextValue(value);
+            input->SelectTextRange(value.Length(), value.Length());
+          }
         }
       }
       // Close the pop-up even if nothing was selected
       ClearSearchTimer();
       ClosePopup();
     }
     // Update last-searched string to the current input, since the input may
     // have changed.  Without this, subsequent backspaces look like text
@@ -1280,30 +1291,63 @@ nsAutoCompleteController::EnterMatch(boo
       // The user wants explicitely to use that result, so this ensures
       // association of the result with the autocompleted text.
       nsAutoString defaultIndexValue;
       if (NS_SUCCEEDED(GetFinalDefaultCompleteValue(defaultIndexValue)))
         value = defaultIndexValue;
     }
 
     if (forceComplete && value.IsEmpty()) {
-      // Since nothing was selected, and forceComplete is specified, that means
-      // we have to find the first default match and enter it instead
+      // See if inputValue is one of the autocomplete results. It can be an
+      // identical value, or if it matched the middle of a result it can be
+      // something like "bar >> foobar" (user entered bar and foobar is
+      // the result value).
+      // If the current search matches one of the autocomplete results, we
+      // should use that result, and not overwrite it with the default value.
+      // It's indeed possible EnterMatch gets called a second time (for example
+      // by the blur handler) and it should not overwrite the current match.
+      nsAutoString inputValue;
+      input->GetTextValue(inputValue);
+      nsAutoString suggestedValue;
+      int32_t pos = inputValue.Find(" >> ");
+      if (pos > 0) {
+        inputValue.Right(suggestedValue, inputValue.Length() - pos - 4);
+      } else {
+        suggestedValue = inputValue;
+      }
+
+      nsAutoString defaultValue;
       for (uint32_t i = 0; i < mResults.Length(); ++i) {
         nsIAutoCompleteResult *result = mResults[i];
+        if (result) {
+          if (defaultValue.IsEmpty()) {
+            int32_t defaultIndex;
+            result->GetDefaultIndex(&defaultIndex);
+            if (defaultIndex >= 0) {
+              result->GetFinalCompleteValueAt(defaultIndex, defaultValue);
+            }
+          }
 
-        if (result) {
-          int32_t defaultIndex;
-          result->GetDefaultIndex(&defaultIndex);
-          if (defaultIndex >= 0) {
-            result->GetFinalCompleteValueAt(defaultIndex, value);
-            break;
+          uint32_t matchCount = 0;
+          result->GetMatchCount(&matchCount);
+          for (uint32_t j = 0; j < matchCount; ++j) {
+            nsAutoString matchValue;
+            result->GetFinalCompleteValueAt(j, matchValue);
+            if (suggestedValue.Equals(matchValue, nsCaseInsensitiveStringComparator())) {
+              value = matchValue;
+              break;
+            }
           }
         }
       }
+      if (value.IsEmpty()) {
+        // Since nothing was selected, and forceComplete is specified, that means
+        // we have to enter the first default match instead.
+        value = defaultValue;
+      }
     }
   }
 
   nsCOMPtr<nsIObserverService> obsSvc =
     mozilla::services::GetObserverService();
   NS_ENSURE_STATE(obsSvc);
   obsSvc->NotifyObservers(input, "autocomplete-will-enter-text", nullptr);
 

--- a/toolkit/content/tests/chrome/test_autocomplete4.xul
+++ b/toolkit/content/tests/chrome/test_autocomplete4.xul
@@ -146,16 +146,24 @@ const tests = [
   },
   { desc: "TAB key should confirm suggestion when forcecomplete is set",
     key: "VK_TAB",
     removeSelection: false,
     forceComplete: true,
     result: "Result",
     start: 6, end: 6
   },
+
+  { desc: "RIGHT key complete from middle",
+    key: "VK_RIGHT",
+    forceComplete: true,
+    completeFromMiddle: true,
+    result: "Result",
+    start: 6, end: 6
+  },
 ];
 
 function nextTest() {
   if (!tests.length) {
     // No more tests to run, finish.
     setTimeout(function() {
       // Unregister the factory so that we don't get in the way of other tests
       componentManager.unregisterFactory(autoCompleteSimpleID, autoCompleteSimple);
@@ -172,30 +180,53 @@ function nextTest() {
   if (currentTest.key == "VK_HOME" && navigator.platform.indexOf("Mac") != -1)
     nextTest();
   else
     setTimeout(runCurrentTest, 0);
 }
 
 function runCurrentTest() {
   var autocomplete = $("autocomplete");
-
   autocomplete.focus();
 
-  synthesizeKey("r", {});
-  synthesizeKey("e", {});
+  if (!currentTest.completeFromMiddle) {
+    synthesizeKey("r", {});
+    synthesizeKey("e", {});
+  }
+  else {
+    synthesizeKey("l", {});
+    synthesizeKey("t", {});
+  }
 }
 
 function searchComplete() {
   var autocomplete = $("autocomplete");
+  autocomplete.setAttribute("forcecomplete", currentTest.forceComplete ? true : false);
+
+  if (currentTest.completeFromMiddle) {
+    if (!currentTest.forceComplete) {
+      synthesizeKey(currentTest.key, {});
+    }
+    else if (!/ >> /.test(autocomplete.value)) {
+      // At this point we should have a value like "lt >> Result" showing.
+      throw new Error("Expected an middle-completed value, got " + autocomplete.value);
+    }
+
+    // For forceComplete a blur should cause a value from the results to get
+    // completed to. E.g. "lt >> Result" will turn into "Result".
+    if (currentTest.forceComplete)
+      autocomplete.blur();
+
+    checkResult();
+    return;
+  }
+
   is(autocomplete.value, "result",
      "Test '" + currentTest.desc + "': autocomplete.value should equal 'result'");
 
-  autocomplete.setAttribute("forcecomplete", currentTest.forceComplete ? true : false);
-
   if (autocomplete.selectionStart == 2) {  // Finished inserting "re" string.
     if (currentTest.removeSelection) {
       // remove current selection
       synthesizeKey("VK_DELETE", {});
     }
 
     synthesizeKey(currentTest.key, {});
 

--- a/toolkit/content/widgets/autocomplete.xml
+++ b/toolkit/content/widgets/autocomplete.xml
@@ -331,22 +331,18 @@
       <method name="attachController">
         <body><![CDATA[
           this.mController.input = this;
         ]]></body>
       </method>
 
       <method name="detachController">
         <body><![CDATA[
-          try {
-            if  (this.mController.input == this)
+          if (this.mController.input == this)
             this.mController.input = null;
-          } catch (ex) {
-            // nothing really to do.
-          }
         ]]></body>
       </method>
 
       <!-- ::::::::::::: popup opening ::::::::::::: -->
 
       <method name="openPopup">
         <body><![CDATA[
           this.popup.openAutocompletePopup(this, this);
@@ -593,18 +589,23 @@
                action="if (this.mController.input == this) this.mController.handleStartComposition();"/>
 
       <handler event="compositionend" phase="capturing"
                action="if (this.mController.input == this) this.mController.handleEndComposition();"/>
 
       <handler event="focus" phase="capturing"
                action="this.attachController();"/>
 
-      <handler event="blur" phase="capturing"
-               action="if (!this._dontBlur) this.detachController();"/>
+      <handler event="blur" phase="capturing"><![CDATA[
+        if (!this._dontBlur) {
+          if (this.forceComplete && this.mController.matchCount >= 1)
+            this.mController.handleEnter(false);
+          this.detachController();
+        }
+      ]]></handler>
     </handlers>
   </binding>
 
   <binding id="autocomplete-result-popup" extends="chrome://global/content/bindings/autocomplete.xml#autocomplete-base-popup">
     <resources>
       <stylesheet src="chrome://global/content/autocomplete.css"/>
       <stylesheet src="chrome://global/skin/tree.css"/>
       <stylesheet src="chrome://global/skin/autocomplete.css"/>