have you gone through How to set the home page

Yes, and as a temporary fix, I set the home page as Google search, but that doesn't correct the Firefox home page.

I even edited the Perf.js to remove all mention of "Yahoo" ,and replaced them with "Google" and then confirmed with "about:config" that Yahoo was gone, but the firefox home page has somehow been modified with hdml code to direct the search to Yahoo. Without using the dreaded "reset" which screws up everything, I don't know how to get a clean copy of the Firefox home page.

Use the Search Reset add-on.

https://addons.mozilla.org/en-US/firefox/addon/searchreset/

As I understand this "search reset" would only restore my default home page, etc. I have manually done all that, and the problem is the default homepage that I have on my computer has actually been altered. I can read the html of it, and it clearly says to go to yahoo.com.

I need a fresh copy of the firefox homepage.

I also just discovered that my edited version of "perfs.js" got changed to "invalidperfs.js" and a new "perfs.js" had the reference back to Yahoo.

I went ahead and tried the "search reset" with no change. I then backed up Firefox again and disabled Session Manager, as I thought it might be carrying stuff over. I then deleted all Sessionstor.js, and all pref.js, and it worked once, and then Yahoo came back.

OH, I also downloaded and installed a fresh copy of Firefox v. 20.0.1

OK, I think I found it. I disabled all of my extensions, deleted the prefs.js file again and then enabled the extensions one by one. When I got to one called Pdforge toolbar (or something like that), I read about it, and don't know why I ever even installed it. I didn't even try to enable it, I just removed it. But I have reinabled all the others and the homepage is staying with Google.

Whoopee!