divx plugin

I use downloadhelper add-on for download videos from several websites, most of videos online are *.flv but some are *.divx and my ffp don't play them, It shows a textbox: "divx plugin", Then ffp searchs the plugin but seems doesn't found anything and then said "unknown plugin".

I have searched that plugin but I don't find it.

Can you tell me where to find that plugin?

Thanks a lot