diff --git a/app/scripts/fetch_xulrunner b/app/scripts/fetch_xulrunner
index 179bcdfaf5..c4404c9ccf 100755
--- a/app/scripts/fetch_xulrunner
+++ b/app/scripts/fetch_xulrunner
@@ -186,6 +186,9 @@ function modify_omni {
 	#rm Telemetry*
 	rm URLDecorationAnnotationsService.sys.mjs
 	popd
+	pushd chrome/toolkit/content/mozapps/extensions
+	rm -rf default-theme
+	popd
 	
 	# Clear most WebExtension manifest properties
 	replace_line 'manifest = normalized.value;' 'manifest = normalized.value;
@@ -232,6 +235,10 @@ function modify_omni {
 	replace_line 'lazy.AddonManagerPrivate.webExtensionsMinPlatformVersion' '"7.0"' modules/addons/AddonUpdateChecker.sys.mjs
 	replace_line 'result.targetApplications.push' 'false && result.targetApplications.push' modules/addons/AddonUpdateChecker.sys.mjs
 	
+	# Prevent automatic installation of default theme
+	# (chrome/toolkit/content/mozapps/extensions/default-theme, removed above)
+	replace_line 'this.maybeInstallBuiltinAddon\(' 'if (false) this.maybeInstallBuiltinAddon(' modules/addons/XPIProvider.jsm
+	
 	# Allow addon installation by bypassing confirmation dialogs. If we want a confirmation dialog,
 	# we need to either add gXPInstallObserver from browser-addons.js [1][2] or provide our own with
 	# Ci.amIWebInstallPrompt [3].