--- a/toolkit/webapps/WebappsInstaller.jsm
+++ b/toolkit/webapps/WebappsInstaller.jsm
@@ -7,16 +7,17 @@ this.EXPORTED_SYMBOLS = ["WebappsInstall
 const Cc = Components.classes;
 const Ci = Components.interfaces;
 const Cu = Components.utils;
 const Cr = Components.results;
 
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/FileUtils.jsm");
 Cu.import("resource://gre/modules/NetUtil.jsm");
+Cu.import("resource://gre/modules/osfile.jsm");
 
 this.WebappsInstaller = {
   /**
    * Creates a native installation of the web app in the OS
    *
    * @param aData the manifest data provided by the web app
    *
    * @returns bool true on success, false if an error was thrown
@@ -313,17 +314,17 @@ WinNativeApp.prototype = {
     uninstaller.copyTo(this.uninstallDir, this.uninstallerFile.leafName);
   },
 
   /**
    * Creates the configuration files into their destination folders.
    */
   _createConfigFiles: function() {
     // ${InstallDir}/webapp.json
-    writeToFile(this.configJson, JSON.stringify(this.webappJson), function() {});
+    writeToFile(this.configJson, JSON.stringify(this.webappJson));
 
     let factory = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                     .getService(Ci.nsIINIParserFactory);
 
     // ${InstallDir}/webapp.ini
     let writer = factory.createINIParser(this.webappINI).QueryInterface(Ci.nsIINIParserWriter);
     writer.setString("Webapp", "Name", this.appName);
     writer.setString("Webapp", "Profile", this.installDir.leafName);
@@ -344,17 +345,17 @@ WinNativeApp.prototype = {
     // ${UninstallDir}/uninstall.log
     let uninstallContent = 
       "File: \\webapp.ini\r\n" +
       "File: \\webapp.json\r\n" +
       "File: \\webapprt.old\r\n" +
       "File: \\chrome\\icons\\default\\default.ico";
     let uninstallLog = this.uninstallDir.clone();
     uninstallLog.append("uninstall.log");
-    writeToFile(uninstallLog, uninstallContent, function() {});
+    writeToFile(uninstallLog, uninstallContent);
   },
 
   /**
    * Writes the keys to the system registry that are necessary for the app operation
    * and uninstall process.
    */
   _writeSystemKeys: function() {
     let parentKey;
@@ -551,17 +552,17 @@ MacNativeApp.prototype = {
     webapprt.append("webapprt-stub");
     webapprt.copyTo(this.macOSDir, "webapprt");
   },
 
   _createConfigFiles: function() {
     // ${ProfileDir}/webapp.json
     let configJson = this.appProfileDir.clone();
     configJson.append("webapp.json");
-    writeToFile(configJson, JSON.stringify(this.webappJson), function() {});
+    writeToFile(configJson, JSON.stringify(this.webappJson));
 
     // ${InstallDir}/Contents/MacOS/webapp.ini
     let applicationINI = this.macOSDir.clone().QueryInterface(Ci.nsILocalFile);
     applicationINI.append("webapp.ini");
 
     let factory = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                     .getService(Ci.nsIINIParserFactory);
 
@@ -595,17 +596,17 @@ MacNativeApp.prototype = {
     <string>0</string>\n\
     <key>FirefoxBinary</key>\n\
 #expand     <string>__MOZ_MACBUNDLE_ID__</string>\n\
   </dict>\n\
 </plist>';
 
     let infoPListFile = this.contentsDir.clone();
     infoPListFile.append("Info.plist");
-    writeToFile(infoPListFile, infoPListContent, function() {});
+    writeToFile(infoPListFile, infoPListContent);
   },
 
   _moveToApplicationsFolder: function() {
     let appDir = Services.dirsvc.get("LocApp", Ci.nsILocalFile);
     let destination = getAvailableFile(appDir,
                                        this.appNameAsFilename,
                                        ".app");
     if (!destination) {
@@ -794,17 +795,17 @@ LinuxNativeApp.prototype = {
       }
     }
 
     return categories;
   },
 
   _createConfigFiles: function() {
     // ${InstallDir}/webapp.json
-    writeToFile(this.configJson, JSON.stringify(this.webappJson), function() {});
+    writeToFile(this.configJson, JSON.stringify(this.webappJson));
 
     let factory = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                     .getService(Ci.nsIINIParserFactory);
 
     let webappsBundle = Services.strings.createBundle("chrome://global/locale/webapps.properties");
 
     // ${InstallDir}/webapp.ini
     let writer = factory.createINIParser(this.webappINI).QueryInterface(Ci.nsIINIParserWriter);
@@ -875,25 +876,21 @@ LinuxNativeApp.prototype = {
 
 /* Helper Functions */
 
 /**
  * Async write a data string into a file
  *
  * @param aFile     the nsIFile to write to
  * @param aData     a string with the data to be written
- * @param aCallback a callback to be called after the process is finished
  */
-function writeToFile(aFile, aData, aCallback) {
-  let ostream = FileUtils.openSafeFileOutputStream(aFile);
-  let converter = Cc["@mozilla.org/intl/scriptableunicodeconverter"]
-                    .createInstance(Ci.nsIScriptableUnicodeConverter);
-  converter.charset = "UTF-8";
-  let istream = converter.convertToInputStream(aData);
-  NetUtil.asyncCopy(istream, ostream, function(x) aCallback(x));
+function writeToFile(aFile, aData) {
+  let path = aFile.path;
+  let data = new TextEncoder().encode(aData);
+  return OS.File.writeAtomic(path, data, { tmpPath: path + ".tmp" });
 }
 
 /**
  * Removes unprintable characters from a string.
  */
 function sanitize(aStr) {
   let unprintableRE = new RegExp("[\\x00-\\x1F\\x7F]" ,"gi");
   return aStr.replace(unprintableRE, "");