{{msg-mw|Fogg-installed|notext=yes}}

Not used indeed:

mw.Firefogg.js: "fogg-installed" : "Firefogg is installed.",
mw.Firefogg.js: "fogg-not-installed" : "Firefogg is not installed or not enabled.",
mw.Firefogg.js: // CSS selector for the "firefogg is installed" message div
mw.Firefogg.js: 'target_installed': false,
mw.Firefogg.js:                 gM( 'fogg-for_improved_uploads' ) + ' ' : gM( 'fogg-not-installed') + ' ';