diff --git a/build/mach_bootstrap.py b/build/mach_bootstrap.py
index 20a58b50fd107..fdb17b1b834f0 100644
--- a/build/mach_bootstrap.py
+++ b/build/mach_bootstrap.py
@@ -247,6 +247,10 @@ def bootstrap(topsrcdir):
         # The resource module is UNIX only.
         pass
 
+    from mozbuild.util import patch_main
+
+    patch_main()
+
     def resolve_repository():
         import mozversioncontrol
 
diff --git a/python/mozbuild/mozbuild/util.py b/python/mozbuild/mozbuild/util.py
index 9917d31fb6f56..071daecc397b0 100644
--- a/python/mozbuild/mozbuild/util.py
+++ b/python/mozbuild/mozbuild/util.py
@@ -1402,6 +1402,71 @@ def recurse_indented_repr(o, level):
     f.write(result)
 
 
+def patch_main():
+    """This is a hack to work around the fact that Windows multiprocessing needs
+    to import the original main module, and assumes that it corresponds to a file
+    ending in .py.
+
+    We do this by a sort of two-level function interposing. The first
+    level interposes forking.get_command_line() with our version defined
+    in my_get_command_line(). Our version of get_command_line will
+    replace the command string with the contents of the fork_interpose()
+    function to be used in the subprocess.
+
+    The subprocess then gets an interposed imp.find_module(), which we
+    hack up to find the main module name multiprocessing will assume, since we
+    know what this will be based on the main module in the parent. If we're not
+    looking for our main module, then the original find_module will suffice.
+
+    See also: http://bugs.python.org/issue19946
+    And: https://bugzilla.mozilla.org/show_bug.cgi?id=914563
+    """
+    # XXX In Python 3.4 the multiprocessing module was re-written and the below
+    # code is no longer valid. The Python issue19946 also claims to be fixed in
+    # this version. It's not clear whether this hack is still needed in 3.4+ or
+    # not, but at least some basic mach commands appear to work without it. So
+    # skip it in 3.4+ until we determine it's still needed.
+    if sys.platform == "win32" and sys.version_info < (3, 4):
+        import os
+        from multiprocessing import forking
+
+        global orig_command_line
+
+        # Figure out what multiprocessing will assume our main module
+        # is called (see python/Lib/multiprocessing/forking.py).
+        main_path = getattr(sys.modules["__main__"], "__file__", None)
+        if main_path is None:
+            # If someone deleted or modified __main__, there's nothing left for
+            # us to do.
+            return
+        main_file_name = os.path.basename(main_path)
+        main_module_name, ext = os.path.splitext(main_file_name)
+        if ext == ".py":
+            # If main is a .py file, everything ought to work as expected.
+            return
+
+        def my_get_command_line():
+            with open(
+                os.path.join(os.path.dirname(__file__), "fork_interpose.py"), "rU"
+            ) as fork_file:
+                fork_code = fork_file.read()
+            # Add our relevant globals.
+            fork_string = (
+                "main_file_name = '%s'\n" % main_file_name
+                + "main_module_name = '%s'\n" % main_module_name
+                + fork_code
+            )
+            cmdline = orig_command_line()
+            # We don't catch errors if "-c" is not found because it's not clear
+            # what we should do if the original command line is not of the form
+            # "python ... -c 'script'".
+            cmdline[cmdline.index("-c") + 1] = fork_string
+            return cmdline
+
+        orig_command_line = forking.get_command_line
+        forking.get_command_line = my_get_command_line
+
+
 def ensure_bytes(value, encoding="utf-8"):
     if isinstance(value, six.text_type):
         return value.encode(encoding)