--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
@@ -1563,17 +1563,21 @@ add_task(async function test_defaultSear
 
   // ... but that if a cohort identifier is set, we send it.
   Services.prefs.setCharPref("browser.search.cohort", "testcohort");
   Services.obs.notifyObservers(null, "browser-search-service", "init-complete");
   data = TelemetryEnvironment.currentEnvironment;
   Assert.equal(data.settings.searchCohort, "testcohort");
 });
 
-add_task(async function test_delayed_defaultBrowser() {
+add_task({ skip_if: () => AppConstants.MOZ_APP_NAME == "thunderbird" },
+         async function test_delayed_defaultBrowser() {
+  // Skip this test on Thunderbird since it is not a browser, so it cannot
+  // be the default browser.
+
   // Make sure we don't have anything already cached for this test.
   await TelemetryEnvironment.testCleanRestart().onInitialized();
 
   let environmentData = TelemetryEnvironment.currentEnvironment;
   checkEnvironmentData(environmentData);
   Assert.equal(environmentData.settings.isDefaultBrowser, null,
                "isDefaultBrowser must be null before the session is restored.");