Resolved strange sidebar / window opens on a few sites on one machine

My machine is running Firefox 3.5.7 in W7 Pro. When I go to a few websites a sidebar of the same window opens on the left side of my screen. It only happens on this one machine, only on a few websites and only in Firefox. I also have another machine with W7 Pro and it doesn't happen on it.

There must be a setting on this machine in Firefox that I have configured somehow but I cannot see what it might be.

Any suggestions gratefully received. Thanks

 

I don't see any Firefox setting for sidebar. However, I have had the experience that the mediaplayerconnectivity addon could open a sidebar. And, that was the addon setting. Since I had problems controlling this action, and the addon was not working as well as desired, I uninstalled it. I have never seen the sidebar since.
Without knowing the type of site, I can't help figure this out.

 

Thanks Westside - I went looking for the mediaplayerconnectivity addon and didn't see it however, I did find a TOC sidbar addon (don't know where it came from) which I disabled and voila, no more sidebar. Thanks for your post