No issue: Add number_of_app_launches custom attribute to nimbus messages.

app/src/main/java/org/mozilla/fenix/gleanplumb/CustomAttributeProvider.kt

@@ -6,6 +6,7 @@ package org.mozilla.fenix.gleanplumb
 
 import android.content.Context
 import org.json.JSONObject
+import org.mozilla.fenix.ext.settings
 import org.mozilla.fenix.utils.BrowsersCache
 import java.text.SimpleDateFormat
 import java.util.Locale
@@ -27,7 +28,8 @@ object CustomAttributeProvider {
         return JSONObject(
             mapOf(
                 "is_default_browser_string" to BrowsersCache.all(context).isDefaultBrowser.toString(),
-                "date_string" to formatter.format(now.time)
+                "date_string" to formatter.format(now.time),
+                "number_of_app_launches" to context.settings().numberOfAppLaunches
             )
         )
     }

nimbus.fml.yaml

@@ -125,6 +125,7 @@ features:
             NEVER: "false"
             I_AM_DEFAULT_BROWSER: "is_default_browser_string == 'true'"
             I_AM_NOT_DEFAULT_BROWSER: "is_default_browser_string == 'false'"
+            USER_ESTABLISHED_INSTALL: "number_of_app_launches >=4"
           actions:
             ENABLE_PRIVATE_BROWSING: ://enable_private_browsing
             INSTALL_SEARCH_WIDGET: ://install_search_widget
@@ -164,7 +165,7 @@ features:
             default-browser:
               text: default_browser_experiment_card_text
               action: "MAKE_DEFAULT_BROWSER"
-              trigger: [ "I_AM_NOT_DEFAULT_BROWSER" ]
+              trigger: [ "I_AM_NOT_DEFAULT_BROWSER","USER_ESTABLISHED_INSTALL" ]
               style: "DEFAULT"
               button-label: preferences_set_as_default_browser