Perhaps you have a Firefox add-on called the Browser Token [1]? It may have been installed with a Skype add-in or independently (presumably it has some connection with eBay).
I saw two people [2], [3] that removing the Browser-Marker add-on does the trick. This is probably not a malicious widget, but simply faulty. Delete it or disconnect and tell us?
If so, you can instruct your users to remove or disable the add-in on your side (in the browser).
. CMS (Wordpress, Joomla ..), , - .
[1] http://www.browserhighlighter.com/
[2] №1
[3] №2