I searched using several engines, nothing found,

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I won't pretend to know much about this but the early part of the link is related to Yahoo. This will give you a bit of an idea - http://www.herdprotect.com/domain-sxp.yimg.com.aspx Along the way you should update your seriously out-of-date Firefox.

What was the result of the malware scan?