When users chose to install the extension via the package manager, it is safe to
assume that they don't want to be bothered by popup messages when there is a new
upstream release of the extension available via other update channels.

Index: keepassxc-browser/keepassxc-browser/background/page.js
===================================================================
--- keepassxc-browser.orig/keepassxc-browser/background/page.js	2020-03-23 12:26:58.921173577 +0100
+++ keepassxc-browser/keepassxc-browser/background/page.js	2020-03-23 12:26:58.921173577 +0100
@@ -8,7 +8,7 @@
     autoReconnect: false,
     autoRetrieveCredentials: true,
     autoSubmit: false,
-    checkUpdateKeePassXC: 3,
+    checkUpdateKeePassXC: 0,
     colorTheme: 'system',
     defaultGroup: '',
     defaultGroupAlwaysAsk: false,