--- a/.eslintignore
+++ b/.eslintignore
@@ -209,16 +209,17 @@ toolkit/components/osfile/**
 # External code:
 toolkit/components/microformats/test/**
 toolkit/components/reader/Readability.js
 toolkit/components/reader/JSDOMParser.js
 
 # Uses preprocessing
 toolkit/content/widgets/videocontrols.xml
 toolkit/content/widgets/wizard.xml
+toolkit/components/extensions/test/mochitest/test_constants.js
 toolkit/components/jsdownloads/src/DownloadIntegration.jsm
 toolkit/components/search/nsSearchService.js
 toolkit/components/url-classifier/**
 toolkit/components/urlformatter/nsURLFormatter.js
 toolkit/identity/FirefoxAccounts.jsm
 toolkit/modules/AppConstants.jsm
 toolkit/mozapps/downloads/nsHelperAppDlg.js
 toolkit/mozapps/extensions/internal/AddonConstants.jsm

--- a/toolkit/components/extensions/moz.build
+++ b/toolkit/components/extensions/moz.build
@@ -17,8 +17,14 @@ EXTRA_JS_MODULES += [
 
 DIRS += ['schemas']
 
 JAR_MANIFESTS += ['jar.mn']
 
 MOCHITEST_MANIFESTS += ['test/mochitest/mochitest.ini']
 MOCHITEST_CHROME_MANIFESTS += ['test/mochitest/chrome.ini']
 XPCSHELL_TESTS_MANIFESTS += ['test/xpcshell/xpcshell.ini']
+
+DEFINES['PYTHON'] = CONFIG['PYTHON']
+OBJDIR_PP_FILES._tests.testing.mochitest.chrome.toolkit.components.extensions.test.mochitest += [
+    'test/mochitest/test_constants.js',
+]
+

new file mode 100644
--- /dev/null
+++ b/toolkit/components/extensions/test/mochitest/test_constants.js
@@ -0,0 +1,6 @@
+#filter substitution
+
+"use strict";
+
+/* global PYTHON */
+this.PYTHON = String.raw`@PYTHON@`;