Make downloads work in FF.

lib/DownloadFile.js

@@ -26,13 +26,21 @@ export default class DownloadFile {
   }
 
   download() {
+
     let blob = this.chunks.toBlob()
     let url = URL.createObjectURL(blob)
+
     let a = document.createElement('a')
+    document.body.appendChild(a)
     a.download = this.name
     a.href = url
     a.click()
+
+    setTimeout(() => {
       URL.revokeObjectURL(url)
+      document.body.removeChild(a)
+    }, 0)
+
   }
 
   toJSON() {