diff --git a/testing/mochitest/runtests.py.in b/testing/mochitest/runtests.py.in
index bc1607b794bd..fdd63cd919d4 100644
--- a/testing/mochitest/runtests.py.in
+++ b/testing/mochitest/runtests.py.in
@@ -149,6 +149,27 @@ class MochitestOptions(optparse.OptionParser):
                     help = "start running tests when the application starts")
     defaults["autorun"] = False
 
+    self.add_option("--total-chunks",
+                    type = "int", dest = "totalChunks",
+                    help = "how many chunks to split the tests up into")
+    defaults["totalChunks"] = None
+
+    self.add_option("--this-chunk",
+                    type = "int", dest = "thisChunk",
+                    help = "which chunk to run")
+    defaults["thisChunk"] = None
+
+    self.add_option("--chunk-by-dir",
+                    type = "int", dest = "chunkByDir",
+                    help = "group tests together in the same chunk that are in the same top chunkByDir directories")
+    defaults["chunkByDir"] = 0
+
+    self.add_option("--shuffle",
+                    dest = "shuffle",
+                    action = "store_true",
+                    help = "randomize test order")
+    defaults["shuffle"] = False
+
     LOG_LEVELS = ("DEBUG", "INFO", "WARNING", "ERROR", "FATAL")
     LEVEL_STRING = ", ".join(LOG_LEVELS)
 
@@ -339,6 +360,13 @@ def main():
   parser = MochitestOptions()
   options, args = parser.parse_args()
 
+  if options.totalChunks is not None and options.thisChunk is None:
+    parser.error("thisChunk must be specified when totalChunks is specified")
+
+  if options.totalChunks:
+    if not 1 <= options.thisChunk <= options.totalChunks:
+       parser.error("thisChunk must be between 1 and totalChunks")
+
   if options.xrePath is None:
     # default xrePath to the app path if not provided
     # but only if an app path was explicitly provided
@@ -426,6 +454,8 @@ Are you executing $objdir/_tests/testing/mochitest/runtests.py?"""
   # autorun -- kick off tests automatically
   # closeWhenDone -- runs quit.js after tests
   # logFile -- logs test run to an absolute path
+  # totalChunks -- how many chunks to split tests into
+  # thisChunk -- which chunk to run
   #
   
   # consoleLevel, fileLevel: set the logging level of the console and
@@ -460,6 +490,13 @@ Are you executing $objdir/_tests/testing/mochitest/runtests.py?"""
       urlOpts.append("fileLevel=" + encodeURIComponent(options.fileLevel))
     if options.consoleLevel:
       urlOpts.append("consoleLevel=" + encodeURIComponent(options.consoleLevel))
+    if options.totalChunks:
+        urlOpts.append("totalChunks=%d" % options.totalChunks)
+        urlOpts.append("thisChunk=%d" % options.thisChunk)
+        if options.chunkByDir:
+            urlOpts.append("chunkByDir=%d" % options.chunkByDir)
+    if options.shuffle:
+        urlOpts.append("shuffle=1")
     if len(urlOpts) > 0:
       testURL += "?" + "&".join(urlOpts)
 
diff --git a/testing/mochitest/tests/SimpleTest/setup.js b/testing/mochitest/tests/SimpleTest/setup.js
index ba2c517f98be..b8a4ecfdfc1c 100644
--- a/testing/mochitest/tests/SimpleTest/setup.js
+++ b/testing/mochitest/tests/SimpleTest/setup.js
@@ -68,7 +68,68 @@ if (!params.quiet) {
 var gTestList = [];
 var RunSet = {}
 RunSet.runall = function(e) {
-  TestRunner.runTests(gTestList);
+  // Which tests we're going to run
+  var my_tests = gTestList;
+
+  if (params.totalChunks && params.thisChunk) {
+    var total_chunks = parseInt(params.totalChunks);
+    // this_chunk is in the range [1,total_chunks]
+    var this_chunk = parseInt(params.thisChunk);
+
+    // We want to split the tests up into chunks according to which directory
+    // they're in
+    if (params.chunkByDir) {
+      var chunkByDir = parseInt(params.chunkByDir);
+      var tests_by_dir = {};
+      var test_dirs = []
+      for (var i = 0; i < gTestList.length; ++i) {
+        var test_path = gTestList[i];
+        if (test_path[0] == '/') {
+          test_path = test_path.substr(1);
+        }
+        var dir = test_path.split("/");
+        // We want the first chunkByDir+1 components, or everything but the
+        // last component, whichever is less.
+        // we add 1 to chunkByDir since 'tests' is always part of the path, and
+        // want to ignore the last component since it's the test filename.
+        dir = dir.slice(0, Math.min(chunkByDir+1, dir.length-1));
+        // reconstruct a directory name
+        dir = dir.join("/");
+        if (!(dir in tests_by_dir)) {
+          tests_by_dir[dir] = [gTestList[i]];
+          test_dirs.push(dir);
+        } else {
+          tests_by_dir[dir].push(gTestList[i]);
+        }
+      }
+      var tests_per_chunk = test_dirs.length / total_chunks;
+      var start = Math.round((this_chunk-1) * tests_per_chunk);
+      var end = Math.round(this_chunk * tests_per_chunk);
+      my_tests = [];
+      var dirs = []
+      for (var i = start; i < end; ++i) {
+        var dir = test_dirs[i];
+        dirs.push(dir);
+        my_tests = my_tests.concat(tests_by_dir[dir]);
+      }
+      TestRunner.logger.log("Running tests in " + dirs.join(", "));
+    } else {
+      var tests_per_chunk = gTestList.length / total_chunks;
+      var start = Math.round((this_chunk-1) * tests_per_chunk);
+      var end = Math.round(this_chunk * tests_per_chunk);
+      my_tests = gTestList.slice(start, end);
+      TestRunner.logger.log("Running tests " + (start+1) + "-" + end + "/" + gTestList.length);
+    }
+  }
+  if (params.shuffle) {
+    for (var i = my_tests.length-1; i > 0; --i) {
+      var j = Math.floor(Math.random() * i);
+      var tmp = my_tests[j];
+      my_tests[j] = my_tests[i];
+      my_tests[i] = tmp;
+    }
+  }
+  TestRunner.runTests(my_tests);
 }
 RunSet.reloadAndRunAll = function(e) {
   e.preventDefault();