Are your familiar with this code injected by firefox?

Question

<div id="atffc">
    <object height="1px" width="1px" id="atff" data="//bin.clearspring.com/at/v/1/button1.6.swf" type="application/x-shockwave-flash">
        <param name="wmode" value="transparent"/><param name="allowScriptAccess" value="always"/>
    </object>
</div>

It only happens the first time you visit.

And I checked there is no such code in source code.

Answer 1

Firefox doesn't add that, it'll be a script on the website you're visiting. Clearspring is:

... the leading provider of social distribution solutions connecting online publishers and advertisers to audiences on the social web. Our free platform, AddThis, powers over 500K web properties, reaching 600 million users worldwide – over half the Internet audience.

Our technologies are used by the largest entertainment companies including NBC, Disney, Fox, MTV and Turner. Leading brands such as Sprint, Verizon, Clearasil, and Snickers leverage our paid performance-based distribution solutions to engage their audiences on Facebook, MySpace, and MyYearbook.

More information might be found on their API Page or in a Google search.