Ça ne répond pas à ta question (ce que fait le plugin) mais ça explique pourquoi il apparaît dans firefox : http://kb.mozillazine.org/Plugin_scanning

For example, if you have Google software such as Google Chrome or Google Earth installed, the Google Update plugin is also installed. The Google Update plugin is then detected in your Mozilla browser via plugin scanning because its directory is specified in the Windows registry for PLIDs. You can disable the Google Update plugin in Firefox 3 using the Add-on Manager.