I think that may be wrong, Raymond. I think this may be the first part of a two part sentence. Have you check the JS source?
I've added this question to a few other questions on Mdale's discussion page.
Hmm not checked, but now. And it seems unused. Maybe doublecheck because I am not very familar with JS
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') + ' ';