diff --git a/src/main.coffee b/src/main.coffee
index 7cf9ab4af..95fcc6f47 100644
--- a/src/main.coffee
+++ b/src/main.coffee
@@ -466,6 +466,8 @@ Main =
 
     if $.hasClass d.body, 'fourchan_x'
       alert '4chan X v2 detected: Disable it or v3 will break.'
+    if /Firefox\/1/.test navigator.userAgent
+      alert 'Your version of Firefox is incompatible with 4chan X v3. Firefox 20+ is required.'
 
     $.event '4chanXInitFinished'
     Main.checkUpdate()