Fix a bug in the message about scheduled drafts at TB shutdown

The warning message that gets displayed when there are scheduled
drafts at TB shutdown said "Thunderbird" where it was supposed to say
"Send Later".

_locales/en/messages.json

@@ -723,11 +723,11 @@
   },
   "ScheduledMessagesWarningQuitRequested": {
     "message": "There are messages scheduled to be sent. Send Later can only send messages when $1 is running. Are you sure you want to exit?",
-    "description": ""
+    "description": "$1 is replaced with the localized name for Thunderbird"
   },
   "ScheduledMessagesWarningQuit": {
     "message": "Send Later can only send messages when $1 is running. Please remember to restart it before your messages are scheduled to be sent!",
-    "description": ""
+    "description": "$1 is replaced with the localized name for Thunderbird"
   },
   "IncompatibleEncryptionTitle": {
     "message": "Encryption settings incompatible with $1",

background.js

@@ -1351,11 +1351,12 @@ const SendLater = {
       await messenger.quitter.removeQuitGrantedObserver();
       return;
     }
-    let appName = messenger.i18n.getMessage("extensionName");
+    let appName = (await messenger.runtime.getBrowserInfo()).name;
+    let extensionName = messenger.i18n.getMessage("extensionName");
     let title =
       messenger.i18n.getMessage("scheduledMessagesWarningTitle") +
       " - " +
-      appName;
+      extensionName;
     let requestWarning = messenger.i18n.getMessage(
       "scheduledMessagesWarningQuitRequested",
       appName,