Bug 1749345 - Skip FirstStartup initting if a previous profile exists. r=nalexander,rhelmer

The thinking here being that if a previous profile exists, then the
--first-startup argument is probably be passed because the user is
reinstalling on a system that still has (or once had) the browser
already installed on it. In that case, we're going to use that
pre-existing profile, and we don't need to do the FirstStartup
things, since they're primarily for systems where a new profile
is being created after install.

Differential Revision: https://phabricator.services.mozilla.com/D199763

Author: mikeconley

browser/components/BrowserContentHandler.sys.mjs

@@ -663,7 +663,10 @@ nsBrowserContentHandler.prototype = {
     }
 
     if (cmdLine.handleFlag("first-startup", false)) {
-      lazy.FirstStartup.init();
+      let override = needHomepageOverride(Services.prefs);
+      if (override == OVERRIDE_NEW_PROFILE) {
+        lazy.FirstStartup.init();
+      }
     }
 
     var fileParam = cmdLine.handleFlagWithParam("file", false);