Another poster mentioned s.yimg.com rearing itself within Firefox. s.yimg.com appears after one has YouTube'd within Firefox.
Googling s.yimg.com coughs up several threads dealing with this - problem.
I fixed by installing LeachBlock [an xpi from Mozilla add-on store]; then blacklisting s.yimg.com within.
Although I haven't tried it yet, BlockSite offers a very similar function within.
I hope this is the correct forum for this solution. It is no longer a problem for me because of LeachBlock.