--- a/toolkit/mozapps/extensions/internal/AddonRepository.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonRepository.jsm
@@ -86,18 +86,17 @@ function convertHTMLToPlainText(html) {
   var converter = Cc["@mozilla.org/widget/htmlformatconverter;1"].
                   createInstance(Ci.nsIFormatConverter);
 
   var input = Cc["@mozilla.org/supports-string;1"].
               createInstance(Ci.nsISupportsString);
   input.data = html.replace(/\n/g, "<br>");
 
   var output = {};
-  converter.convert("text/html", input, input.data.length, "text/unicode",
-                    output, {});
+  converter.convert("text/html", input, "text/unicode", output);
 
   if (output.value instanceof Ci.nsISupportsString)
     return output.value.data.replace(/\r\n/g, "\n");
   return html;
 }
 
 async function getAddonsToCache(aIds) {
   let types = Preferences.get(PREF_GETADDONS_CACHE_TYPES) || DEFAULT_CACHE_TYPES;

--- a/widget/nsHTMLFormatConverter.cpp
+++ b/widget/nsHTMLFormatConverter.cpp
@@ -97,25 +97,24 @@ nsHTMLFormatConverter::CanConvert(const 
 // accessible from JS. Currently, this only accepts HTML input, so anything else is invalid.
 //
 //XXX This method copies the data WAAAAY too many time for my liking. Grrrrrr. Mostly it's because
 //XXX we _must_ put things into nsStrings so that the parser will accept it. Lame lame lame lame. We
 //XXX also can't just get raw unicode out of the nsString, so we have to allocate heap to get
 //XXX unicode out of the string. Lame lame lame.
 //
 NS_IMETHODIMP
-nsHTMLFormatConverter::Convert(const char *aFromDataFlavor, nsISupports *aFromData, uint32_t aDataLen,
-                               const char *aToDataFlavor, nsISupports **aToData, uint32_t *aDataToLen)
+nsHTMLFormatConverter::Convert(const char *aFromDataFlavor, nsISupports *aFromData,
+                               const char *aToDataFlavor, nsISupports **aToData)
 {
-  if ( !aToData || !aDataToLen )
+  if ( !aToData )
     return NS_ERROR_INVALID_ARG;
 
   nsresult rv = NS_OK;
   *aToData = nullptr;
-  *aDataToLen = 0;
 
   if ( !nsCRT::strcmp(aFromDataFlavor, kHTMLMime) ) {
     nsAutoCString toFlavor ( aToDataFlavor );
 
     // HTML on clipboard is going to always be double byte so it will be in a primitive
     // class of nsISupportsString. Also, since the data is in two byte chunks the
     // length represents the length in 1-byte chars, so we need to divide by two.
     nsCOMPtr<nsISupportsString> dataWrapper0 ( do_QueryInterface(aFromData) );
@@ -127,36 +126,30 @@ nsHTMLFormatConverter::Convert(const cha
     dataWrapper0->GetData ( dataStr );  // COPY #1
     // note: conversion to text/plain is done inside the clipboard. we do not need to worry
     // about it here.
     if ( toFlavor.Equals(kHTMLMime) || toFlavor.Equals(kUnicodeMime) ) {
       nsresult res;
       if (toFlavor.Equals(kHTMLMime)) {
         int32_t dataLen = dataStr.Length() * 2;
         nsPrimitiveHelpers::CreatePrimitiveForData ( toFlavor, dataStr.get(), dataLen, aToData );
-        if ( *aToData )
-          *aDataToLen = dataLen;
       } else {
         nsAutoString outStr;
         res = ConvertFromHTMLToUnicode(dataStr, outStr);
         if (NS_SUCCEEDED(res)) {
           int32_t dataLen = outStr.Length() * 2;
           nsPrimitiveHelpers::CreatePrimitiveForData ( toFlavor, outStr.get(), dataLen, aToData );
-          if ( *aToData )
-            *aDataToLen = dataLen;
         }
       }
     } // else if HTML or Unicode
     else if ( toFlavor.Equals(kAOLMailMime) ) {
       nsAutoString outStr;
       if ( NS_SUCCEEDED(ConvertFromHTMLToAOLMail(dataStr, outStr)) ) {
         int32_t dataLen = outStr.Length() * 2;
         nsPrimitiveHelpers::CreatePrimitiveForData ( toFlavor, outStr.get(), dataLen, aToData );
-        if ( *aToData )
-          *aDataToLen = dataLen;
       }
     } // else if AOL mail
     else {
       rv = NS_ERROR_FAILURE;
     }
   } // if we got html mime
   else
     rv = NS_ERROR_FAILURE;

--- a/widget/nsIFormatConverter.idl
+++ b/widget/nsIFormatConverter.idl
@@ -34,17 +34,17 @@ interface nsIFormatConverter : nsISuppor
 
   /**
     * Converts from one flavor to another.
     *
     * @param  aFromFormatConverter flavor to convert from
     * @param  aFromFormatConverter flavor to convert to (destination own the memory)
     * @returns returns NS_OK if it was converted
     */
-  void convert ( in string aFromDataFlavor, in nsISupports aFromData, in unsigned long aDataLen,
-                   in string aToDataFlavor, out nsISupports aToData, out unsigned long aDataToLen ) ; 
+  void convert ( in string aFromDataFlavor, in nsISupports aFromData,
+                   in string aToDataFlavor, out nsISupports aToData ) ; 
 
 };
 
 
 %{ C++
 
 %}

--- a/widget/nsTransferable.cpp
+++ b/widget/nsTransferable.cpp
@@ -302,19 +302,18 @@ nsTransferable::GetTransferData(const ch
           rv = dataProvider->GetFlavorData(this, aFlavor,
                                            getter_AddRefs(dataBytes));
           if (NS_FAILED(rv)) {
             // Give up.
             return rv;
           }
         }
 
-        uint32_t dataLen;
-        return mFormatConv->Convert(data.GetFlavor().get(), dataBytes, 0,
-                                    aFlavor, aData, &dataLen);
+        return mFormatConv->Convert(data.GetFlavor().get(), dataBytes, aFlavor,
+                                    aData);
       }
     }
   }
 
   return NS_ERROR_FAILURE;
 }
 
 //
@@ -363,23 +362,20 @@ nsTransferable::SetTransferData(const ch
   if (mFormatConv) {
     for (size_t i = 0; i < mDataArray.Length(); ++i) {
       DataStruct& data = mDataArray.ElementAt(i);
       bool canConvert = false;
       mFormatConv->CanConvert(aFlavor, data.GetFlavor().get(), &canConvert);
 
       if (canConvert) {
         nsCOMPtr<nsISupports> ConvertedData;
-        uint32_t ConvertedLen;
         mFormatConv->Convert(aFlavor,
                              aData,
-                             0,
                              data.GetFlavor().get(),
-                             getter_AddRefs(ConvertedData),
-                             &ConvertedLen);
+                             getter_AddRefs(ConvertedData));
         data.SetData(ConvertedData, mPrivateData);
         return NS_OK;
       }
     }
   }
 
   // Can't set data neither directly nor through converter. Just add this flavor
   // and try again