diff --git a/LICENSE b/LICENSE
deleted file mode 100644
index 15dc0a9..0000000
--- a/LICENSE
+++ /dev/null
@@ -1,22 +0,0 @@
-The MIT License (MIT)
-
-Copyright (c) 2015 Sebastian Jørgensen
-
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
-
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
-
diff --git a/README.md b/README.md
index 4eb5f9f..e4ee995 100644
--- a/README.md
+++ b/README.md
@@ -1,10 +1,19 @@
 # js-untar
 Library for extracting tar files in the browser.
 
+## Browser feature requirements
+* [ArrayBuffer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer).
+* [Web Workers](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API).
+* [Blob](https://developer.mozilla.org/en-US/docs/Web/API/Blob) and the [Blob() constructor](https://developer.mozilla.org/en-US/docs/Web/API/Blob/Blob).
+
+As of September 2015 this includes Chrome>=20, Firefox>=13, IE>=10, Opera>=12.10 and Safari>=8.
+[Web Worker transfarable objects](https://developer.mozilla.org/en-US/docs/Web/API/Worker/postMessage) are used when available, increasing speed greatly. This is supported in Chrome>=21, Firefox>=18, Opera>=15 and Safari.
+
 ## Documentation
-Load the module with RequireJS or similar. Module is a function that returns a modified Promise with a progress callback. 
+Load the module with RequireJS or similar. The module is a function that returns a modified Promise with a progress callback. 
 This callback is executed every time a file is extracted. 
-The standard Promise.then method is also called when extraction is done, with all extracted files as argument.
+The standard Promise.then method is also called when extraction is done, with all extracted files as argument. 
+The extraction is done in a [Web Worker](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API) to allow the main UI thread to continue.
 
 ### Example:
 
@@ -12,6 +21,7 @@ The standard Promise.then method is also called when extraction is done, with al
 		// Load the source ArrayBuffer from a XMLHttpRequest or any other way.
 		var sourceBuffer = ...;
 		
+		// Listening to progress events
 		untar(sourceBuffer)
 			.progress(function(extractedFile) {
 				...
@@ -19,10 +29,22 @@ The standard Promise.then method is also called when extraction is done, with al
 			.then(function(extractedFiles) {
 				...
 			});
+
+		untar(sourceBuffer).then(
+			function(extractedFiles) { // onSuccess
+				...
+			},
+			function(err) { // onError
+				...
+			},
+			function(extractedFile) { // onProgress
+				...
+			}
+		);
 	});
 
 ### File object
-The returned file object has the following properties. Most of these are explained in the [Tar wikipedia entry](https://en.wikipedia.org/wiki/Tar_(computing)#File_format).
+The returned file object(s) has the following properties. Most of these are explained in the [Tar wikipedia entry](https://en.wikipedia.org/wiki/Tar_(computing)#File_format).
 
 * name = The full filename (including path and ustar filename prefix).
 * mode
diff --git a/karma.conf.js b/karma.conf.js
index ad44733..885e2d4 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -15,6 +15,7 @@ module.exports = function(config) {
 
     // list of files / patterns to load in the browser
     files: [
+      'https://www.promisejs.org/polyfills/promise-6.1.0.js',
       {pattern: 'build/**/**/*.js', included: false},
       {pattern: 'spec/**/*.*', included: false},
       'test-main.js'
@@ -57,7 +58,7 @@ module.exports = function(config) {
 
     // start these browsers
     // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
-    browsers: ['Chrome'],
+    browsers: ['Chrome', 'Firefox', 'IE'],
 
     browserNoActivityTimeout: 60000,
 
diff --git a/package.json b/package.json
index a8891e3..5a31c0d 100644
--- a/package.json
+++ b/package.json
@@ -39,6 +39,8 @@
     "jasmine-core": "^2.3.4",
     "karma": "^0.13.9",
     "karma-chrome-launcher": "^0.2.0",
+    "karma-firefox-launcher": "^0.1.6",
+    "karma-ie-launcher": "^0.2.0",
     "karma-jasmine": "^0.3.6",
     "karma-phantomjs-launcher": "^0.2.1",
     "karma-requirejs": "^0.2.2",
diff --git a/src/ProgressivePromise.js b/src/ProgressivePromise.js
index 846fd22..3f68e96 100644
--- a/src/ProgressivePromise.js
+++ b/src/ProgressivePromise.js
@@ -37,13 +37,15 @@ function ProgressivePromise(fn) {
 		return promise;
 	};
 
+	var origThen = promise.then;
+
 	promise.then = function(onSuccess, onFail, onProgress) {
-		Promise.prototype.then.call(promise, onSuccess, onFail);
+		origThen.call(promise, onSuccess, onFail);
 
 		if (onProgress !== undefined) {
 			promise.progress(onProgress);
 		}
-		
+
 		return promise;
 	};