This is a cookie problem, as this ugly green “Sign Up” button appears in any browser that you never used to sign in to Facebook.
I had the same problem in different browsers, and if you try to clear your cookies in your favorite browser, you will see a green button again, regardless of browser.
If you use the likebox plugin in iframe mode, this can be avoided by discarding the parameters of the public profile, that is, by setting "stream" and "show_faces" to false.
This is an iframe example that works:
<iframe scrolling="no" frameborder="0" allowtransparency="true" style="border:none; overflow:hidden; width:245px; height:75px;" src="http://www.facebook.com/plugins/likebox.php?href=http%3A%2F%2Fwww.facebook.com%2Fonglesetpaillettes81 &width=245&height=100&colorscheme=dark&show_faces=false&border_color=%23D4A7AD&stream=false&header=false"> </iframe>
Hope this helps.
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