Help please with a box that keeps popping up every time I change web pages?

A little box comes up like this:-

Opening

You have chosen to open st.

which is a application/octel-stream from

http://ad.jieldmanager.com/

What should Firefox do with this file

Open with (Browse)

or

Save File

do this automatically for files like this from now on

ok or cancel.

I seem to be using alot of my credit on my vodafone dongle and wondering if this has anything to do with it. Many thanks in advance.