--- a/dom/media/test/mochitest.ini
+++ b/dom/media/test/mochitest.ini
@@ -604,16 +604,17 @@ skip-if = (os == 'mac' && os_version == 
 [test_bug686942.html]
 [test_bug726904.html]
 [test_bug874897.html]
 [test_bug879717.html]
 tags=capturestream
 skip-if = os == 'win' && !debug # bug 1140675
 [test_bug883173.html]
 [test_bug895091.html]
+tags=webvtt
 [test_bug895305.html]
 [test_bug919265.html]
 [test_bug957847.html]
 [test_bug1018933.html]
 [test_bug1113600.html]
 tags=capturestream
 [test_bug1242338.html]
 [test_bug1242594.html]

--- a/dom/media/webvtt/vtt.jsm
+++ b/dom/media/webvtt/vtt.jsm
@@ -1431,16 +1431,21 @@ this.EXPORTED_SYMBOLS = ["WebVTT"];
 
         var line;
         if (self.state === "HEADER") {
           line = parseHeader();
         }
 
         while (self.buffer) {
           if (!line) {
+            // Since the data receiving is async, we need to wait until the
+            // buffer gets the full line.
+            if (!/\r\n|\n/.test(self.buffer)) {
+              return this;
+            }
             line = collectNextLine();
           }
 
           switch (self.state) {
           case "ID":
             // Ignore NOTE and line terminator
             if (/^NOTE($|[ \t])/.test(line) || !line) {
               break;