boxes with smileys spread over the screen when FB is open

Hello,

Since a while I have a strange blocks with some smileys and emoticons spread over the desktop. This happens only when my Firefox is running and Facebook is open.

The smileys and emoticons that are in these boxes are nothing that I tend to use.

Norton doesn't find any Virus, other virus checkers I tried, also an online esset, didn't find any infection either.

Does anybody knows this is a virus or something else?

Did somebody encounter this already?

Many thanks in advance!