Firefox for Android is no longer available for download in the T-Store in South Korea. You can still download it for free and receive updates from Google Play.
Where can I download Firefox in South Korea?
Google Play lets you install Firefox and keep it up to date. See Install Firefox on an Android device using Google Play for more information.
I've already installed Firefox through the T-Store
To continue receiving the latest updates, uninstall Firefox then reinstall it through Google Play.
Recommended: Before you uninstall Firefox, set up Firefox Accounts on your device to upload and sync your bookmarks, passwords and browsing information across all of your devices. See Sync bookmarks, tabs, history and passwords on Android for more information.