was added automatically at the end of the post.
<img style="border: medium none"
src="image/png;base64,iVBORw0KGgoAAAANS ......
First thought was some trojan/virus. After looking deeper into it, I found it's only a Vista Aero(Firefox theme) error.
And that code is nothing but this image:
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To fix this issue, either remove Vista Aero or update it to the latest version.