diff -r 000000000000 -r 6474c204b198 layout/tools/reftest/print-manifest-dirs.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/tools/reftest/print-manifest-dirs.py	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,79 @@
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import sys, os.path, re
+
+commentRE = re.compile(r"\s+#")
+conditionsRE = re.compile(r"^(fails|needs-focus|random|skip|asserts|slow|require-or|silentfail|pref|test-pref|ref-pref|fuzzy)")
+httpRE = re.compile(r"HTTP\((\.\.(\/\.\.)*)\)")
+protocolRE = re.compile(r"^\w+:")
+
+def parseManifest(manifest, dirs):
+  """Parse the reftest manifest |manifest|, adding all directories containing
+  tests (and the dirs containing the manifests themselves) to the set |dirs|."""
+  manifestdir = os.path.dirname(os.path.abspath(manifest))
+  dirs.add(manifestdir)
+  f = file(manifest)
+  urlprefix = ''
+  for line in f:
+    if line[0] == '#':
+      continue # entire line was a comment
+    m = commentRE.search(line)
+    if m:
+      line = line[:m.start()]
+    line = line.strip()
+    if not line:
+      continue
+    items = line.split()
+    while conditionsRE.match(items[0]):
+      del items[0]
+    if items[0] == "HTTP":
+      del items[0]
+    m = httpRE.match(items[0])
+    if m:
+      # need to package the dir referenced here
+      d = os.path.normpath(os.path.join(manifestdir, m.group(1)))
+      dirs.add(d)
+      del items[0]
+
+    if items[0] == "url-prefix":
+      urlprefix = items[1]
+      continue
+    elif items[0] == "default-preferences":
+      continue
+    elif items[0] == "include":
+      parseManifest(os.path.join(manifestdir, items[1]), dirs)
+      continue
+    elif items[0] == "load" or items[0] == "script":
+      testURLs = [items[1]]
+    elif items[0] == "==" or items[0] == "!=":
+      testURLs = items[1:3]
+    for u in testURLs:
+      m = protocolRE.match(u)
+      if m:
+        # can't very well package about: or data: URIs
+        continue
+      d = os.path.dirname(os.path.normpath(os.path.join(manifestdir, urlprefix + u)))
+      dirs.add(d)
+  f.close()
+
+def printTestDirs(topsrcdir, topmanifests):
+  """Parse |topmanifests| and print a list of directories containing the tests
+  within (and the manifests including those tests), relative to |topsrcdir|."""
+  topsrcdir = os.path.abspath(topsrcdir)
+  dirs = set()
+  for manifest in topmanifests:
+    parseManifest(manifest, dirs)
+  for dir in sorted(dirs):
+    d = dir[len(topsrcdir):].replace('\\','/')
+    if d[0] == '/':
+      d = d[1:]
+    print d
+
+if __name__ == '__main__':
+  if len(sys.argv) < 3:
+    print >>sys.stderr, "Usage: %s topsrcdir reftest.list [reftest.list]*" % sys.argv[0]
+    sys.exit(1)
+  printTestDirs(sys.argv[1], sys.argv[2:])