how to synchronize contacts from desktop to smartphone android

The Thunderbird mobile app doesn't have it's own address book or sync mechanism for Android contacts. You will need to look into third party android apps. If your email provider supports carddav, an app such as Davx5 might be a possible solution. '''F-Droid:''' https://f-droid.org/packages/at.bitfire.davdroid/ '''Play store:''' https://play.google.com/store/apps/details?id=at.bitfire.davdroid&hl=en-US The idea for a built-in address book has been requested by the community on [https://connect.mozilla.org/t5/ideas/idb-p/ideas/label-name/thunderbird%20android mozilla connect]. You can show support by voting for this, or raise your own ideas and suggestions. https://connect.mozilla.org/t5/ideas/an-address-book-in-thunderbird-for-android/idi-p/84952