diff --git a/testing/profiles/perf/user.js b/testing/profiles/perf/user.js
index cb9eb1ea8504..7736d4505e8d 100644
--- a/testing/profiles/perf/user.js
+++ b/testing/profiles/perf/user.js
@@ -66,7 +66,7 @@ user_pref("identity.fxaccounts.auth.uri", "https://127.0.0.1/fxa-dummy/");
 user_pref("identity.fxaccounts.migrateToDevEdition", false);
 // Make tests run consistently on DevEdition (which has a lightweight theme
 // selected by default).
-user_pref("lightweightThemes.selectedThemeID", "default-theme@mozilla.org");
+user_pref("lightweightThemes.selectedThemeID", "");
 user_pref("media.capturestream_hints.enabled", true);
 user_pref("media.gmp-manager.url", "http://127.0.0.1/gmpmanager-dummy/update.xml");
 // Don't block old libavcodec libraries when testing, because our test systems
diff --git a/testing/profiles/unittest/user.js b/testing/profiles/unittest/user.js
index 2fd9d60fa355..67c35ed37a03 100644
--- a/testing/profiles/unittest/user.js
+++ b/testing/profiles/unittest/user.js
@@ -189,7 +189,7 @@ user_pref("layout.css.shape-outside.enabled", true);
 user_pref("layout.spammy_warnings.enabled", false);
 // Make tests run consistently on DevEdition (which has a lightweight theme
 // selected by default).
-user_pref("lightweightThemes.selectedThemeID", "default-theme@mozilla.org");
+user_pref("lightweightThemes.selectedThemeID", "");
 // Disable all recommended Marionette preferences for Gecko tests.
 // The prefs recommended by Marionette are typically geared towards
 // consumer automation; not vendor testing.
diff --git a/toolkit/components/extensions/test/browser/browser_ext_management_themes.js b/toolkit/components/extensions/test/browser/browser_ext_management_themes.js
index bca911328601..4d81dc89eab7 100644
--- a/toolkit/components/extensions/test/browser/browser_ext_management_themes.js
+++ b/toolkit/components/extensions/test/browser/browser_ext_management_themes.js
@@ -125,7 +125,6 @@ add_task(async function test_management_themes() {
   };
   is(await extension.awaitMessage("onInstalled"), "Bling", "LWT installed");
   is(await extension.awaitMessage("onEnabled"), "Bling", "LWT enabled");
-  is(await extension.awaitMessage("onDisabled"), "Default", "default disabled");
 
   await theme.startup();
   is(await extension.awaitMessage("onInstalled"), "Simple theme test", "webextension theme installed");