Bug with Firefox 5.0.1?

Since I upgraded to Firefox 5.0.1. I see this error in Firebug:

Se ha escrito un árbol no equilibrado usando document.write() lo que ha provocado que los datos de red se hayan reinterpretado ("reparsed"). Para más información, https://developer.mozilla.org/en/Optimizing_Your_Pages_for_Speculat...

 

The error references to the line that loads btbuckets.js. Into this file I have:

//<![CDATA[[
$BTB={s:XXXXX};
$BTB.jsh="btbuckets.com/bt.js";
//]]>

$BTB.jshost = (document.location.protocol == "https:")?"https://ssl.":"http://static.";
document.write(unescape("%3Cscript src='" + $BTB.jshost + $BTB.jsh + "' type='text/javascript'%3E%3C/script%3E"));

 

 

Any idea how to solve it?

 

Thanks a lot