diff --git a/download_firefox_nightly.py b/download_firefox_nightly.py
index 9a1f983..bb3d96b 100755
--- a/download_firefox_nightly.py
+++ b/download_firefox_nightly.py
@@ -5,11 +5,14 @@
 
 """Downloads a Firefox Nightly build for the current platform."""
 
+import datetime
+import glob
 import os
 import shutil
 import sys
 import subprocess
 import tarfile
+import time
 import zipfile
 
 from optparse import OptionParser
@@ -23,6 +26,87 @@
 from mozdownload import scraper
 
 
+def _Touch(a_file):
+  with open(a_file, 'a'):
+    os.utime(a_file, None)
+
+
+def _FindFallbackFirefoxBuild(target_dir):
+  firefox_archives = glob.glob(os.path.join(target_dir, '*tar.bz2'))
+  if not firefox_archives:
+    return None
+
+  firefox_archives.sort(key=os.path.getmtime, reverse=True)
+  newest_build = firefox_archives[0]
+  build_age_seconds = time.time() - os.path.getmtime(newest_build)
+  build_age_days = datetime.timedelta(seconds=build_age_seconds).days
+
+  return newest_build, build_age_days
+
+
+def _MaybeDownload(target_dir, force):
+  try:
+    downloader = scraper.DailyScraper(directory=target_dir, version=None)
+    filename = downloader.build_filename(downloader.binary)
+    firefox_archive = os.path.join(target_dir, filename)
+
+    if os.path.exists(firefox_archive) and not force:
+      # Touch the file anyway since we were 'successful', so we can accurately
+      # compute the age of the most recent download attempt and act accordingly
+      # when a download fails later.
+      _Touch(firefox_archive)
+      print 'Skipping download as %s is already downloaded.' % firefox_archive
+      return None
+
+    downloader.download()
+    print 'Downloaded %s' % firefox_archive
+    return firefox_archive
+  except scraper.NotFoundException as exception:
+    print 'Failed to download firefox: %s.' % exception
+    fallback_build, age_days = _FindFallbackFirefoxBuild(target_dir)
+
+    if not fallback_build:
+      raise Exception('We failed to download Firefox and we have no builds to '
+                      'fall back on; failing...')
+    if age_days > 3:
+      raise Exception('We have failed to download firefox builds for more '
+                      'than 3 days now: failing so someone looks at it. The '
+                      'most recent build we have is %d days old.' % age_days)
+
+    print 'Using %s instead; it is %d days old.' % (fallback_build, age_days)
+    return fallback_build
+
+
+def _ExtractArchive(firefox_archive, target_dir):
+  # TODO(phoglund): implement on win/mac. See http://crbug.com/454303.
+  if sys.platform == 'darwin':
+    print "Disabled on mac..."
+    return 0
+    volume = '/Volumes/Nightly'
+    firefox_executable = '%s/FirefoxNightly.app' % target_dir
+
+    # Unmount any previous downloads.
+    subprocess.call(['hdiutil', 'detach', volume])
+
+    subprocess.check_call(['hdiutil', 'attach', firefox_archive])
+    shutil.copytree('%s/FirefoxNightly.app' % volume, firefox_executable)
+    subprocess.check_call(['hdiutil', 'detach', volume])
+  elif sys.platform == 'linux2':
+    tar_archive = tarfile.open(firefox_archive, 'r:bz2')
+    tar_archive.extractall(path=target_dir)
+  elif sys.platform == 'win32':
+    print "Disabled on win..."
+    return 0
+    zip_archive = zipfile.ZipFile(firefox_archive)
+    zip_archive.extractall(path=target_dir)
+  else:
+    print >> sys.stderr, 'Unsupported platform: %s' % sys.platform
+    return 1
+
+  print 'Extracted %s' % firefox_archive
+  return 0
+
+
 def main():
   usage = 'usage: %prog -t <target_dir>'
   parser = OptionParser(usage)
@@ -40,45 +124,9 @@
   if not os.path.isdir(target_dir):
     os.mkdir(target_dir)
 
-  downloader = scraper.DailyScraper(directory=target_dir, version=None)
-  firefox_archive = os.path.join(target_dir,
-                                 downloader.build_filename(downloader.binary))
-
-  if os.path.exists(firefox_archive) and not options.force:
-    print 'Skipping download as %s is already downloaded.' % firefox_archive
-    return 0
-
-  downloader.download()
-  print 'Downloaded %s' % firefox_archive
-
-  # Extract the archive.
-  # TODO(phoglund): implement on win/mac
-  if sys.platform == 'darwin':
-    print "Temporarily disabled on mac..."
-    return 0
-    volume = '/Volumes/Nightly'
-    firefox_executable = '%s/FirefoxNightly.app' % target_dir
-
-    # Unmount any previous downloads.
-    subprocess.call(['hdiutil', 'detach', volume])
-
-    subprocess.check_call(['hdiutil', 'attach', firefox_archive])
-    shutil.copytree('%s/FirefoxNightly.app' % volume, firefox_executable)
-    subprocess.check_call(['hdiutil', 'detach', volume])
-  elif sys.platform == 'linux2':
-    tar_archive = tarfile.open(firefox_archive, 'r:bz2')
-    tar_archive.extractall(path=target_dir)
-  elif sys.platform == 'win32':
-    print "Temporarily disabled on win..."
-    return 0
-    zip_archive = zipfile.ZipFile(firefox_archive)
-    zip_archive.extractall(path=target_dir)
-  else:
-    print >> sys.stderr, 'Unsupported platform: %s' % sys.platform
-    return 1
-  
-  print 'Extracted %s' % firefox_archive
-  return 0
+  firefox_archive = _MaybeDownload(target_dir, options.force)
+  if firefox_archive:
+    return _ExtractArchive(firefox_archive, target_dir)
 
 if __name__ == '__main__':
   sys.exit(main())