diff --git a/config/makefiles/xpidl/Makefile.in b/config/makefiles/xpidl/Makefile.in
index 6f45864da419..c0495883f0de 100644
--- a/config/makefiles/xpidl/Makefile.in
+++ b/config/makefiles/xpidl/Makefile.in
@@ -41,7 +41,6 @@ code_gen_deps := $(topsrcdir)/xpcom/reflect/xptinfo/perfecthash.py
 	$(PYTHON_PATH) $(PLY_INCLUDE) -I$(topsrcdir)/xpcom/idl-parser -I$(DEPTH)/xpcom/idl-parser/xpidl \
 		$(process_py) --cache-dir $(DEPTH)/xpcom/idl-parser/xpidl --depsdir $(idl_deps_dir) \
 		--bindings-conf $(topsrcdir)/dom/bindings/Bindings.conf \
-		$(foreach dir,$($(basename $(notdir $@))_dirs),--input-dir $(dir)) \
 		$(foreach dir,$(all_idl_dirs),-I $(dir)) \
 		$(dist_include_dir) $(dist_xpcrs_dir) $(@D) \
 		$(basename $(notdir $@)) $($(basename $(notdir $@))_deps)
diff --git a/python/mozbuild/mozbuild/action/xpidl-process.py b/python/mozbuild/mozbuild/action/xpidl-process.py
index bdf0f64761bf..ae9ba873eed4 100755
--- a/python/mozbuild/mozbuild/action/xpidl-process.py
+++ b/python/mozbuild/mozbuild/action/xpidl-process.py
@@ -28,7 +28,7 @@ from mozbuild.util import FileAvoidWrite
 
 
 def process(input_dirs, inc_paths, bindings_conf, cache_dir, header_dir,
-            xpcrs_dir, xpt_dir, deps_dir, module, stems):
+            xpcrs_dir, xpt_dir, deps_dir, module, idl_files):
     p = IDLParser(outputdir=cache_dir)
 
     xpts = []
@@ -43,17 +43,10 @@ def process(input_dirs, inc_paths, bindings_conf, cache_dir, header_dir,
     # up to date, we will not re-process XPIDL files if the processor changes.
     rule.add_dependencies(iter_modules_in_path(topsrcdir))
 
-    def read_stem(stem):
-        idl = '%s.idl' % stem
-        for p in input_dirs:
-            idl_file = os.path.join(topsrcdir, p, idl)
-            if os.path.exists(idl_file):
-                return idl_file, open(idl_file).read()
-
-        raise Exception('Could not find stem file for %s' % idl)
-
-    for stem in stems:
-        path, idl_data = read_stem(stem)
+    for path in idl_files:
+        basename = os.path.basename(path)
+        stem, _ = os.path.splitext(basename)
+        idl_data = open(path).read()
 
         idl = p.parse(idl_data, filename=path)
         idl.resolve(inc_paths, p, webidlconfig)
@@ -111,7 +104,7 @@ def main(argv):
     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.')
+        help='Source .idl file(s).')
     parser.add_argument('-I', dest='incpath', action='append', default=[],
         help='Extra directories where to look for included .idl files.')
 
diff --git a/python/mozbuild/mozbuild/backend/common.py b/python/mozbuild/mozbuild/backend/common.py
index 1c4d2e438c54..aaa01ee82f58 100644
--- a/python/mozbuild/mozbuild/backend/common.py
+++ b/python/mozbuild/mozbuild/backend/common.py
@@ -86,7 +86,7 @@ class XPIDLManager(object):
         # can be found.  Yes, we have XPIDL modules with files from
         # multiple directories.
         t = self.modules.setdefault(entry['module'], (set(), set()))
-        t[0].add(entry['root'])
+        t[0].add(entry['source'])
         t[1].add(dirname)
 
 class BinariesCollection(object):
diff --git a/python/mozbuild/mozbuild/backend/recursivemake.py b/python/mozbuild/mozbuild/backend/recursivemake.py
index c27a1414b415..43e7b95c7856 100644
--- a/python/mozbuild/mozbuild/backend/recursivemake.py
+++ b/python/mozbuild/mozbuild/backend/recursivemake.py
@@ -1056,7 +1056,6 @@ class RecursiveMakeBackend(CommonBackend):
             sources, directories = modules[module]
             all_directories |= directories
             deps = sorted(sources)
-            directories = sorted(directories)
 
             # It may seem strange to have the .idl files listed as
             # prerequisites both here and in the auto-generated .pp files.
@@ -1070,8 +1069,6 @@ class RecursiveMakeBackend(CommonBackend):
             # an mtime newer than the .xpt, it will trigger xpt generation.
             mk.add_statement('%s_deps = %s' % (module, ' '.join(deps)))
 
-            mk.add_statement('%s_dirs = %s' % (module, ' '.join(directories)))
-
             build_files.add_optional_exists('%s.xpt' % module)
 
         mk.add_statement('all_idl_dirs = %s' % ' '.join(sorted(all_directories)))
diff --git a/python/mozbuild/mozbuild/backend/tup.py b/python/mozbuild/mozbuild/backend/tup.py
index dc99cfc99018..a282a0e47912 100644
--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -790,7 +790,7 @@ class TupBackend(CommonBackend):
         all_idl_directories.update(*map(lambda x: x[1], manager.modules.itervalues()))
 
         all_xpts = []
-        for module, (idls, directories) in sorted(manager.modules.iteritems()):
+        for module, (idls, _) in sorted(manager.modules.iteritems()):
             cmd = [
                 '$(PYTHON_PATH)',
                 '$(PLY_INCLUDE)',
@@ -801,8 +801,6 @@ class TupBackend(CommonBackend):
                 '--bindings-conf', '$(topsrcdir)/dom/bindings/Bindings.conf',
             ]
 
-            for d in directories:
-                cmd.extend(['--input-dir', d])
             for d in all_idl_directories:
                 cmd.extend(['-I', d])
 
@@ -816,9 +814,10 @@ class TupBackend(CommonBackend):
 
             all_xpts.append('$(MOZ_OBJ_ROOT)/%s/%s.xpt' % (backend_file.relobjdir, module))
             outputs = ['%s.xpt' % module]
-            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/include/%s.h' % f for f in sorted(idls)])
-            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/xpcrs/rt/%s.rs' % f for f in sorted(idls)])
-            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/xpcrs/bt/%s.rs' % f for f in sorted(idls)])
+            stems = sorted(mozpath.splitext(mozpath.basename(idl))[0] for idl in idls)
+            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/include/%s.h' % f for f in stems])
+            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/xpcrs/rt/%s.rs' % f for f in stems])
+            outputs.extend(['$(MOZ_OBJ_ROOT)/dist/xpcrs/bt/%s.rs' % f for f in stems])
             backend_file.rule(
                 inputs=[
                     '$(MOZ_OBJ_ROOT)/xpcom/idl-parser/xpidl/xpidllex.py',