I checked all of those places and found nothing that is easily identifiable as belonging to Yahoo.
I remembered that I had an external hard drive with a system image on it that I had not used in at least a year. I reasoned that, if this toolbar was added as an extension or plug-in that had been installed in the last 3-6 months, if I booted from the external hard drive it would not show up.
I booted from the external drive, started Safari and went to My.Yahoo and signed in and the extra toolbar showed up.
From this I conclude that, unlike some other extra toolbars I have seen discussed, this extra toolbar is a hard-coded feature of the My.Yahoo pages and is not done using extensions or plug-ins that could be removed by the end user.
I think I need to extend my search for a Yahoo setting or "preference" to tweak.
If that is the case, it would seem I should redirect my discussion of this issue to some sort of Yahoo forum.
Do you concur? I will start to locate whatever Yahoo forums I can (and if I find the answer, will come back here and post a link). In the meantime, do you have any particular suggestions as to where I should look?