--- a/config/config.mk
+++ b/config/config.mk
@@ -617,18 +617,23 @@ endif
 ifneq (WINNT_,$(OS_ARCH)_$(GNU_CC))
 LIBS_DIR	= -L$(DIST)/bin -L$(DIST)/lib
 ifdef LIBXUL_SDK
 LIBS_DIR	+= -L$(LIBXUL_SDK)/bin -L$(LIBXUL_SDK)/lib
 endif
 endif
 
 # Default location of include files
+ifndef LIBXUL_SDK
 IDL_PARSER_DIR = $(topsrcdir)/xpcom/idl-parser
 IDL_PARSER_CACHE_DIR = $(DEPTH)/xpcom/idl-parser
+else
+IDL_PARSER_DIR = $(LIBXUL_SDK)/sdk/bin
+IDL_PARSER_CACHE_DIR = $(LIBXUL_SDK)/sdk/bin
+endif
 
 SDK_LIB_DIR = $(DIST)/sdk/lib
 SDK_BIN_DIR = $(DIST)/sdk/bin
 
 DEPENDENCIES	= .md
 
 MOZ_COMPONENT_LIBS=$(XPCOM_LIBS) $(MOZ_COMPONENT_NSPR_LIBS)
 

--- a/config/makefiles/xpidl/Makefile.in
+++ b/config/makefiles/xpidl/Makefile.in
@@ -44,16 +44,20 @@ dist_include_dir := $(DIST)/include
 process_py := $(topsrcdir)/python/mozbuild/mozbuild/action/xpidl-process.py
 
 # TODO we should use py_action, but that would require extra directories to be
 # in the virtualenv.
 idlprocess := $(PYTHON_PATH) $(PLY_INCLUDE) -I$(IDL_PARSER_DIR) -I$(IDL_PARSER_CACHE_DIR) \
     $(process_py) --cache-dir $(IDL_PARSER_CACHE_DIR) $(dist_idl_dir) \
         $(dist_include_dir) $(idl_xpt_dir) $(idl_deps_dir)
 
+ifdef LIBXUL_SDK
+idlprocess += -I$(LIBXUL_SDK)/idl
+endif
+
 xpidl_modules := @xpidl_modules@
 
 @xpidl_rules@
 
 linked_xpt_files := $(addprefix $(idl_xpt_dir)/,$(addsuffix .xpt,$(xpidl_modules)))
 depends_files := $(foreach root,$(xpidl_modules),$(idl_deps_dir)/$(root).pp)
 
 GARBAGE += $(linked_xpt_files) $(depends_files)

--- a/js/src/config/config.mk
+++ b/js/src/config/config.mk
@@ -617,18 +617,23 @@ endif
 ifneq (WINNT_,$(OS_ARCH)_$(GNU_CC))
 LIBS_DIR	= -L$(DIST)/bin -L$(DIST)/lib
 ifdef LIBXUL_SDK
 LIBS_DIR	+= -L$(LIBXUL_SDK)/bin -L$(LIBXUL_SDK)/lib
 endif
 endif
 
 # Default location of include files
+ifndef LIBXUL_SDK
 IDL_PARSER_DIR = $(topsrcdir)/xpcom/idl-parser
 IDL_PARSER_CACHE_DIR = $(DEPTH)/xpcom/idl-parser
+else
+IDL_PARSER_DIR = $(LIBXUL_SDK)/sdk/bin
+IDL_PARSER_CACHE_DIR = $(LIBXUL_SDK)/sdk/bin
+endif
 
 SDK_LIB_DIR = $(DIST)/sdk/lib
 SDK_BIN_DIR = $(DIST)/sdk/bin
 
 DEPENDENCIES	= .md
 
 MOZ_COMPONENT_LIBS=$(XPCOM_LIBS) $(MOZ_COMPONENT_NSPR_LIBS)
 

--- a/moz.build
+++ b/moz.build
@@ -21,11 +21,13 @@ if not CONFIG['LIBXUL_SDK']:
     if CONFIG['MOZ_MEMORY']:
         add_tier_dir('base', ['memory'])
 
     if not CONFIG['MOZ_NATIVE_ZLIB']:
         add_tier_dir('base', ['modules/zlib'])
 
     add_tier_dir('base', ['mozglue', 'memory/mozalloc'])
 
+add_tier_dir('precompile', 'xpcom/xpidl')
+
 # Bring in the configuration for the configured application.
 if CONFIG['COMPILE_ENVIRONMENT']:
     include('/' + CONFIG['MOZ_BUILD_APP'] + '/app.mozbuild')

--- a/python/mozbuild/mozbuild/action/xpidl-process.py
+++ b/python/mozbuild/mozbuild/action/xpidl-process.py
@@ -19,33 +19,33 @@ from typelib import write_typelib
 from xpidl import IDLParser
 from xpt import xpt_link
 
 from mozbuild.makeutil import Makefile
 from mozbuild.pythonutil import iter_modules_in_path
 from mozbuild.util import FileAvoidWrite
 
 
-def process(input_dir, cache_dir, header_dir, xpt_dir, deps_dir, module, stems):
+def process(input_dir, inc_paths, cache_dir, header_dir, xpt_dir, deps_dir, module, stems):
     p = IDLParser(outputdir=cache_dir)
 
     xpts = {}
     mk = Makefile()
     rule = mk.create_rule()
 
     # Write out dependencies for Python modules we import. If this list isn't
     # up to date, we will not re-process XPIDL files if the processor changes.
     rule.add_dependencies(iter_modules_in_path(topsrcdir))
 
     for stem in stems:
         path = os.path.join(input_dir, '%s.idl' % stem)
         idl_data = open(path).read()
 
         idl = p.parse(idl_data, filename=path)
-        idl.resolve([input_dir], p)
+        idl.resolve([input_dir] + inc_paths, p)
 
         header_path = os.path.join(header_dir, '%s.h' % stem)
         deps_path = os.path.join(deps_dir, '%s.pp' % stem)
 
         xpt = BytesIO()
         write_typelib(idl, xpt, path)
         xpt.seek(0)
         xpts[stem] = xpt
@@ -76,15 +76,17 @@ def main(argv):
     parser.add_argument('xptdir',
         help='Directory in which to write xpt file.')
     parser.add_argument('depsdir',
         help='Directory in which to write dependency files.')
     parser.add_argument('module',
         help='Final module name to use for linked output xpt file.')
     parser.add_argument('idls', nargs='+',
         help='Source .idl file(s). Specified as stems only.')
+    parser.add_argument('-I', dest='incpath', action='append', default=[],
+        help='Extra directories where to look for included .idl files.')
 
     args = parser.parse_args(argv)
-    process(args.inputdir, args.cache_dir, args.headerdir, args.xptdir,
-        args.depsdir, args.module, args.idls)
+    process(args.inputdir, args.incpath, args.cache_dir, args.headerdir,
+        args.xptdir, args.depsdir, args.module, args.idls)
 
 if __name__ == '__main__':
     main(sys.argv[1:])

--- a/xpcom/moz.build
+++ b/xpcom/moz.build
@@ -1,17 +1,16 @@
 # -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # 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/.
 
 PARALLEL_DIRS += [
     'idl-parser',
-    'xpidl',
 ]
 
 DIRS += [
     'typelib',
     'string',
     'glue',
     'base',
     'ds',

--- a/xpcom/xpidl/Makefile.in
+++ b/xpcom/xpidl/Makefile.in
@@ -1,11 +1,13 @@
 # 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/.
 
 export::
+ifndef LIBXUL_SDK
 	$(call SUBMAKE,xpidl-parser,$(DEPTH)/xpcom/idl-parser)
+endif
 	$(call py_action,process_install_manifest,$(DIST)/idl $(DEPTH)/_build_manifests/install/dist_idl)
 	$(call SUBMAKE,xpidl,$(DEPTH)/config/makefiles/xpidl)
 
 clean clobber realclean clobber_all distclean::
 	$(call SUBMAKE,$@,$(DEPTH)/config/makefiles/xpidl)