--- a/toolkit/webapps/WebappsInstaller.jsm
+++ b/toolkit/webapps/WebappsInstaller.jsm
@@ -340,17 +340,17 @@ WinNativeApp.prototype = {
    */
   install: function(aZipPath) {
     return Task.spawn(function() {
       yield this._getInstallDir();
 
       try {
         yield this._createDirectoryStructure();
         yield this._copyPrebuiltFiles();
-        this._createConfigFiles();
+        yield this._createConfigFiles();
 
         if (aZipPath) {
           yield OS.File.move(aZipPath, OS.Path.join(this.tmpInstallDir,
                                                     "application.zip"));
         }
 
         yield this.getIcon();
 
@@ -474,18 +474,18 @@ WinNativeApp.prototype = {
                        OS.Path.join(this.tmpInstallDir, this.uninstallerFile));
   },
 
   /**
    * Creates the configuration files into their destination folders.
    */
   _createConfigFiles: function() {
     // ${InstallDir}/webapp.json
-    writeToFile(OS.Path.join(this.tmpInstallDir, this.configJson),
-                JSON.stringify(this.webappJson));
+    yield writeToFile(OS.Path.join(this.tmpInstallDir, this.configJson),
+                      JSON.stringify(this.webappJson));
 
     let factory = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                     .getService(Ci.nsIINIParserFactory);
 
     // ${InstallDir}/webapp.ini
     let webappINIfile = getFile(this.tmpInstallDir, this.webappINI);
 
     let writer = factory.createINIParser(webappINIfile)
@@ -510,19 +510,19 @@ WinNativeApp.prototype = {
       "File: \\webapp.ini\r\n" +
       "File: \\webapp.json\r\n" +
       "File: \\webapprt.old\r\n" +
       "File: \\chrome\\icons\\default\\default.ico";
     if (this.isPackaged) {
       uninstallContent += "\r\nFile: \\application.zip";
     }
 
-    writeToFile(OS.Path.join(this.tmpInstallDir, this.uninstallDir,
-                             "uninstall.log"),
-                uninstallContent);
+    yield writeToFile(OS.Path.join(this.tmpInstallDir, this.uninstallDir,
+                                   "uninstall.log"),
+                      uninstallContent);
   },
 
   /**
    * Writes the keys to the system registry that are necessary for the app operation
    * and uninstall process.
    */
   _writeSystemKeys: function() {
     let parentKey;
@@ -670,17 +670,17 @@ MacNativeApp.prototype = {
 
   install: function(aZipPath) {
     return Task.spawn(function() {
       yield this._getInstallDir();
 
       try {
         yield this._createDirectoryStructure();
         this._copyPrebuiltFiles();
-        this._createConfigFiles();
+        yield this._createConfigFiles();
 
         if (aZipPath) {
           yield OS.File.move(aZipPath, OS.Path.join(this.tmpInstallDir,
                                                     "application.zip"));
         }
 
         yield this.getIcon();
 
@@ -756,18 +756,18 @@ MacNativeApp.prototype = {
   _copyPrebuiltFiles: function() {
     let destDir = getFile(this.tmpInstallDir, this.macOSDir);
     let stub = getFile(this.runtimeFolder, "webapprt-stub");
     stub.copyTo(destDir, "webapprt");
   },
 
   _createConfigFiles: function() {
     // ${ProfileDir}/webapp.json
-    writeToFile(OS.Path.join(this.appProfileDir, "webapp.json"),
-                JSON.stringify(this.webappJson));
+    yield writeToFile(OS.Path.join(this.appProfileDir, "webapp.json"),
+                      JSON.stringify(this.webappJson));
 
     // ${InstallDir}/Contents/MacOS/webapp.ini
     let applicationINI = getFile(this.tmpInstallDir, this.macOSDir, "webapp.ini");
 
     let writer = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                    .getService(Ci.nsIINIParserFactory)
                    .createINIParser(applicationINI)
                    .QueryInterface(Ci.nsIINIParserWriter);
@@ -803,18 +803,18 @@ MacNativeApp.prototype = {
     <true/>\n\
     <key>NSPrincipalClass</key>\n\
     <string>GeckoNSApplication</string>\n\
     <key>FirefoxBinary</key>\n\
 #expand     <string>__MOZ_MACBUNDLE_ID__</string>\n\
   </dict>\n\
 </plist>';
 
-    writeToFile(OS.Path.join(this.tmpInstallDir, this.contentsDir, "Info.plist"),
-                infoPListContent);
+    yield writeToFile(OS.Path.join(this.tmpInstallDir, this.contentsDir, "Info.plist"),
+                      infoPListContent);
   },
 
   /**
    * Process the icon from the imageStream as retrieved from
    * the URL by getIconForApp(). This will bundle the icon to the
    * app package at Contents/Resources/appicon.icns.
    *
    * @param aMimeType     the icon mimetype
@@ -874,17 +874,17 @@ LinuxNativeApp.prototype = {
 
   install: function(aZipPath) {
     return Task.spawn(function() {
       this._getInstallDir();
 
       try {
         this._createDirectoryStructure();
         this._copyPrebuiltFiles();
-        this._createConfigFiles();
+        yield this._createConfigFiles();
 
         if (aZipPath) {
           yield OS.File.move(aZipPath, OS.Path.join(this.tmpInstallDir,
                                                     "application.zip"));
         }
 
         yield this.getIcon();
 
@@ -990,18 +990,18 @@ LinuxNativeApp.prototype = {
       }
     }
 
     return categories;
   },
 
   _createConfigFiles: function() {
     // ${InstallDir}/webapp.json
-    writeToFile(OS.Path.join(this.tmpInstallDir, this.configJson),
-                JSON.stringify(this.webappJson));
+    yield writeToFile(OS.Path.join(this.tmpInstallDir, this.configJson),
+                      JSON.stringify(this.webappJson));
 
     let webappsBundle = Services.strings.createBundle("chrome://global/locale/webapps.properties");
 
     // ${InstallDir}/webapp.ini
     let webappINIfile = getFile(this.tmpInstallDir, this.webappINI);
 
     let writer = Cc["@mozilla.org/xpcom/ini-processor-factory;1"]
                    .getService(Ci.nsIINIParserFactory)
@@ -1085,19 +1085,25 @@ LinuxNativeApp.prototype = {
  * Async write a data string into a file
  *
  * @param aPath     the path to the file to write to
  * @param aData     a string with the data to be written
  */
 function writeToFile(aPath, aData) {
   return Task.spawn(function() {
     let data = new TextEncoder().encode(aData);
-    let file = yield OS.File.open(aPath, { truncate: true }, { unixMode: PERMS_FILE });
-    yield file.write(data);
-    yield file.close();
+
+    let file;
+    try {
+      file = yield OS.File.open(aPath, { truncate: true, write: true },
+                                { unixMode: PERMS_FILE });
+      yield file.write(data);
+    } finally {
+      yield file.close();
+    }
   });
 }
 
 /**
  * Removes unprintable characters from a string.
  */
 function sanitize(aStr) {
   let unprintableRE = new RegExp("[\\x00-\\x1F\\x7F]" ,"gi");