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?

I'm in safe mode now. It took me several tries just to scroll down to this box.

I've scanned my computer with AVG, Malware Bytes, and Spybot and found no detectable malware or viruses.

So yes, the problem is still here.

This is just with Firefox, yes?

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

It does not happen on YouTube but does happen on Facebook, Twitter and this site. Also in some forums.

I have not noticed in in gmail but sometimes in my startmail it does it. I have not noticed it in Yahoo mail either.

Now it's not doing it. The only difference I can think of is that:

1. I just restarted FF again, but not in safe mode 2. I am not using the mouse, only touchpad.

Probably have to go home and test it with the mouse again to see if it repeats. Then I'll probably know.

Keep us posted.

It was indeed the wireless mouse. FF is not at fault.

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.