hi, please try the version from https://addons.mozilla.org/firefox/addon/yahoo-toolbar/

Thank you VERY MUCH, Philipp. It worked beautifully. :-)

Yahoo Toolbar is signed but it appears to be behaving in a way that invalidates its signature for some users. The Mozilla Addon team will loop in Yahoo to help diagnose this issue. Hopefully this will be resolved quickly.

In the meantime you could enable unsigned addons, but this introduces risks of malicious addons.

Thanks

It just happens so that every time I get out of Firefox and then start it again I have to install Yahoo Toolbar. It's a bit of a pain, but it works. Maybe this is so because I get all cookies wiped out when I close Firefox.

Followup: The Yahoo toolbar add-on has been updated. The latest version fixes the problem. If you changed the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config page) as a workaround, you should reset the preference back to true.