--- a/modules/libpref/nsIPrefBranch.idl
+++ b/modules/libpref/nsIPrefBranch.idl
@@ -94,44 +94,71 @@ interface nsIPrefBranch : nsISupports
    * @see setCharPref
    */
   [optional_argc,binaryname(GetFloatPrefWithDefault)]
   float getFloatPref(in string aPrefName, [optional] in float aDefaultValue);
   [noscript,binaryname(GetFloatPref)]
   float getFloatPrefXPCOM(in string aPrefName);
 
   /**
-   * Called to get the state of an individual string preference.
+   * Called to get the state of an individual ascii string preference.
    *
    * @param aPrefName The string preference to retrieve.
    * @param aDefaultValue The string to return if the preference is not set.
    *
    * @return string   The value of the requested string preference.
    *
    * @see setCharPref
    */
   [optional_argc,binaryname(GetCharPrefWithDefault)]
   string getCharPref(in string aPrefName, [optional] in string aDefaultValue);
   [noscript,binaryname(GetCharPref)]
   string getCharPrefXPCOM(in string aPrefName);
 
   /**
-   * Called to set the state of an individual string preference.
+   * Called to set the state of an individual ascii string preference.
    *
    * @param aPrefName The string preference to set.
    * @param aValue    The string value to set the preference to.
    *
    * @throws Error if setting failed or the preference has a default
              value of a type other than string.
    *
    * @see getCharPref
    */
   void setCharPref(in string aPrefName, in string aValue);
 
   /**
+   * Called to get the state of an individual unicode string preference.
+   *
+   * @param aPrefName The string preference to retrieve.
+   * @param aDefaultValue The string to return if the preference is not set.
+   *
+   * @return string   The value of the requested string preference.
+   *
+   * @see setStringPref
+   */
+  [optional_argc]
+  AUTF8String getStringPref(in string aPrefName,
+                            [optional] in AUTF8String aDefaultValue);
+
+  /**
+   * Called to set the state of an individual unicode string preference.
+   *
+   * @param aPrefName The string preference to set.
+   * @param aValue    The string value to set the preference to.
+   *
+   * @throws Error if setting failed or the preference has a default
+             value of a type other than string.
+   *
+   * @see getStringPref
+   */
+  void setStringPref(in string aPrefName, in AUTF8String aValue);
+
+  /**
    * Called to get the state of an individual integer preference.
    *
    * @param aPrefName The integer preference to get the value of.
    * @param aDefaultValue The value to return if the preference is not set.
    *
    * @return long     The value of the requested integer preference.
    *
    * @see setIntPref
@@ -159,16 +186,17 @@ interface nsIPrefBranch : nsISupports
    * preference is a preference which represents an XPCOM object that can not
    * be easily represented using a standard boolean, integer or string value.
    *
    * @param aPrefName The complex preference to get the value of.
    * @param aType     The XPCOM interface that this complex preference
    *                  represents. Interfaces currently supported are:
    *                    - nsIFile
    *                    - nsISupportsString (UniChar)
+   *                      (deprecated; see getStringPref)
    *                    - nsIPrefLocalizedString (Localized UniChar)
    * @param aValue    The XPCOM object into which to the complex preference 
    *                  value should be retrieved.
    *
    * @throws Error The value does not exist or is the wrong type.
    *
    * @see setComplexValue
    */
@@ -180,16 +208,17 @@ interface nsIPrefBranch : nsISupports
    * preference is a preference which represents an XPCOM object that can not
    * be easily represented using a standard boolean, integer or string value.
    *
    * @param aPrefName The complex preference to set the value of.
    * @param aType     The XPCOM interface that this complex preference
    *                  represents. Interfaces currently supported are:
    *                    - nsIFile
    *                    - nsISupportsString (UniChar)
+   *                      (deprecated; see setStringPref)
    *                    - nsIPrefLocalizedString (Localized UniChar)
    * @param aValue    The XPCOM object from which to set the complex preference 
    *                  value.
    *
    * @throws Error if setting failed or the value is the wrong type.
    *
    * @see getComplexValue
    */

--- a/modules/libpref/nsPrefBranch.cpp
+++ b/modules/libpref/nsPrefBranch.cpp
@@ -237,16 +237,46 @@ nsresult nsPrefBranch::SetCharPrefIntern
 {
   ENSURE_MAIN_PROCESS("Cannot SetCharPref from content process:", aPrefName);
   NS_ENSURE_ARG(aPrefName);
   NS_ENSURE_ARG(aValue);
   const char *pref = getPrefName(aPrefName);
   return PREF_SetCharPref(pref, aValue, mIsDefault);
 }
 
+NS_IMETHODIMP nsPrefBranch::GetStringPref(const char *aPrefName,
+                                          const nsACString& aDefaultValue,
+                                          uint8_t _argc,
+                                          nsACString& _retval)
+{
+  nsXPIDLCString utf8String;
+  nsresult rv = GetCharPref(aPrefName, getter_Copies(utf8String));
+  if (NS_SUCCEEDED(rv)) {
+    _retval = utf8String;
+    return rv;
+  }
+
+  if (_argc == 1) {
+    _retval = aDefaultValue;
+    return NS_OK;
+  }
+
+  return rv;
+}
+
+NS_IMETHODIMP nsPrefBranch::SetStringPref(const char *aPrefName, const nsACString& aValue)
+{
+  nsresult rv = CheckSanityOfStringLength(aPrefName, aValue);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  return SetCharPrefInternal(aPrefName, PromiseFlatCString(aValue).get());
+}
+
 NS_IMETHODIMP nsPrefBranch::GetIntPrefWithDefault(const char *aPrefName,
                                                   int32_t aDefaultValue,
                                                   uint8_t _argc, int32_t *_retval)
 {
   nsresult rv = GetIntPref(aPrefName, _retval);
 
   if (NS_FAILED(rv) && _argc == 1) {
     *_retval = aDefaultValue;
@@ -420,16 +450,20 @@ nsresult nsPrefBranch::CheckSanityOfStri
   }
   return CheckSanityOfStringLength(aPrefName, strlen(aValue));
 }
 
 nsresult nsPrefBranch::CheckSanityOfStringLength(const char* aPrefName, const nsAString& aValue) {
   return CheckSanityOfStringLength(aPrefName, aValue.Length());
 }
 
+nsresult nsPrefBranch::CheckSanityOfStringLength(const char* aPrefName, const nsACString& aValue) {
+  return CheckSanityOfStringLength(aPrefName, aValue.Length());
+}
+
 nsresult nsPrefBranch::CheckSanityOfStringLength(const char* aPrefName, const uint32_t aLength) {
   if (aLength > MAX_PREF_LENGTH) {
     return NS_ERROR_ILLEGAL_VALUE;
   }
   if (aLength <= MAX_ADVISABLE_PREF_LENGTH) {
     return NS_OK;
   }
   nsresult rv;

--- a/modules/libpref/nsPrefBranch.h
+++ b/modules/libpref/nsPrefBranch.h
@@ -207,16 +207,17 @@ protected:
     , mFreeingObserverList(false)
   {}
 
   nsresult   GetDefaultFromPropertiesFile(const char *aPrefName, char16_t **return_buf);
   // As SetCharPref, but without any check on the length of |aValue|
   nsresult   SetCharPrefInternal(const char *aPrefName, const char *aValue);
   // Reject strings that are more than 1Mb, warn if strings are more than 16kb
   nsresult   CheckSanityOfStringLength(const char* aPrefName, const nsAString& aValue);
+  nsresult   CheckSanityOfStringLength(const char* aPrefName, const nsACString& aValue);
   nsresult   CheckSanityOfStringLength(const char* aPrefName, const char* aValue);
   nsresult   CheckSanityOfStringLength(const char* aPrefName, const uint32_t aLength);
   void RemoveExpiredCallback(PrefCallback *aCallback);
   const char *getPrefName(const char *aPrefName);
   void       freeObserverList(void);
 
 private:
   int32_t               mPrefRootLength;

--- a/modules/libpref/test/unit/test_defaultValues.js
+++ b/modules/libpref/test/unit/test_defaultValues.js
@@ -23,16 +23,32 @@ function run_test() {
   do_check_throws(function() { ps.getCharPref(prefName); },
                   Cr.NS_ERROR_UNEXPECTED);
   strictEqual(ps.getCharPref(prefName, ""), "");
   strictEqual(ps.getCharPref(prefName, "string"), "string");
   ps.setCharPref(prefName, "foo");
   strictEqual(ps.getCharPref(prefName), "foo");
   strictEqual(ps.getCharPref(prefName, "string"), "foo");
 
+  prefName = "test.default.values.string";
+  do_check_throws(function() { ps.getCharPref(prefName); },
+                  Cr.NS_ERROR_UNEXPECTED);
+  strictEqual(ps.getStringPref(prefName, ""), "");
+  strictEqual(ps.getStringPref(prefName, "éèçàê€"), "éèçàê€");
+  ps.setStringPref(prefName, "éèçàê€");
+  strictEqual(ps.getStringPref(prefName), "éèçàê€");
+  strictEqual(ps.getStringPref(prefName, "string"), "éèçàê€");
+  strictEqual(ps.getStringPref(prefName),
+              ps.getComplexValue(prefName, Ci.nsISupportsString).data);
+  let str = Cc["@mozilla.org/supports-string;1"].
+              createInstance(Ci.nsISupportsString);
+  str.data = "ù€ÚîœïŒëøÇ“";
+  ps.setComplexValue(prefName, Ci.nsISupportsString, str);
+  strictEqual(ps.getStringPref(prefName), "ù€ÚîœïŒëøÇ“");
+
   prefName = "test.default.values.float";
   do_check_throws(function() { ps.getFloatPref(prefName); },
                   Cr.NS_ERROR_UNEXPECTED);
   strictEqual(ps.getFloatPref(prefName, 3.5), 3.5);
   strictEqual(ps.getFloatPref(prefName, 0), 0);
   ps.setCharPref(prefName, 1.75);
   strictEqual(ps.getFloatPref(prefName), 1.75);
   strictEqual(ps.getFloatPref(prefName, 3.5), 1.75);