Where does the focus go? Are you using a mouse pad? Disable it.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Start your Computer in safe mode with networking. Then start Firefox. Try Safe websites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Thanks for the reply.

I have no idea where the focus goes - there are no other windows/diaglog boxes open.

I assume that by "mouse pad" you mean graphics tablet? I do have a Wacom tablet attached to my PC. I will try disconnecting it.

I am talking about the finger pad that some use in place of a mouse. Some are known to send signals even when the pad is not being used.

So I tried installing the adminscope focus logger. Whenever I run the logger at startup, I am unable to recreate the focus stealing fault. What I do see is a Sandboxie process (which I have installed) and sometimes an explorer process. However neither of these processes appear to create the focus stealing problem.