Set browser.download.manager.openDelay to 0 by default. We've been testing with a half second for a while and the lack of feedback is confusing people. It might be annoying to have a download manager appear for instantly done downloads but the effect is if you don't show the download manager that nothing has happened, which is worse (given the options that exist for turning off the download manager)

browser/app/profile/firefox.js

@@ -87,7 +87,7 @@ pref("browser.download.manager.retention", 2);
 pref("browser.download.manager.showWhenStarting", true);
 pref("browser.download.manager.useWindow", true);
 pref("browser.download.manager.closeWhenDone", true);
-pref("browser.download.manager.openDelay", 500);
+pref("browser.download.manager.openDelay", 0);
 pref("browser.download.manager.focusWhenStarting", false);
 pref("browser.download.manager.flashCount", 2);