Rated 5 out of 5 stars
Made an account just to post this reply, hope it helps others.
I was having the same issue as ListerUK with both FF 21 and FF 22. I went to Custom New Tab's options menu, checked "Place focus in URL bar", UNchecked it again, saved the options, and voila, problem solved!
I don't think it was an add-on issue, but something to do with FF reinitializing the existing add-ons when it installed its updates.
Been using CNT for a looonnng time. Cheers, Ed!!