Use .overrideMimeType() for Firefox XHR

By overriding the mime type of the default XML to 'text/css' we get rid
of the automatic XML parsing and DOM building that XMLHttpRequest tries
to do whenever it gets a response.

lib/less/browser.js

@@ -167,6 +167,9 @@ function xhr(url, callback, errback) {
     var xhr = getXMLHttpRequest();
     var async = isFileProtocol ? false : less.async;
 
+    if (typeof xhr.overrideMimeType === 'function') {
+      xhr.overrideMimeType('text/css');
+    }
     xhr.open('GET', url, async);
     xhr.send(null);