First, look in Add/Remove Programs in the Control Panel for Mcafee Siteadvisor, and uninstall it if present. Next, look in Firefox's add ons and press uninstall on Siteadvisor. After restarting Firefox, download the older version
http://community.mcafee.com/showthread.php?t=220931&page=3 (The link is in the second post on that page). Save the file somewhere like your desktop, and drag it into an open Firefox window. It should install, but Firefox will say there is the update available. DON'T INSTALL THE UPDATE.
Instead, find the folder where Firefox saves the extension by searching for "{1650a312-02bc-40ee-977e-83f158701739}" on your computer. It should be a folder,and it should contain a file called install.rdf. Open this file in a text editor and delete the line
<em:updateURL>https://sadownload.mcafee.com/products/sa/firefox/update.rdf</em:updateURL>
Make sure Firefox is closed and save the file. Now when you open Firefox again, check for updates, and it should say none available for Siteadvisor. This will keep Siteadvisor at version 26.6 without ever updating because the update server url is deleted.