hi, you can try this: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named browser.newtabpage.activity-stream.topSitesCount. double-click it and change its value to 24 or some other multiple of 6...

Thanks! That worked. Now I just need to try and remember the addresses for all of the missing sites.

try restarting firefox once - maybe they'll come back on their own...

Yep! Restarting solved it. Thanks again.