Thanks for reaching out to the Community! Sorry you are having some issues with the subject line personalization.
I took a look at your last campaign and the only thing I see that you left out was the quote marks around the fallback text. So instead of [[FIRSTNAME OR Greetings]], use [[FIRSTNAME OR "Greetings"]]. Other than that it should be working. You can insert any of our standard contact record fields, or as you saw, a custom field too. Custom fields are set up by you if you want to store some data in the contact record that does not fit the definition of one of our standard fields. You can find more information in this article.
I wanted to reach back out to inform you on recent updates we've made. We now have a feature that allows you to easily personalize the subject line of your emails! You can click here to see how to use this new personalization feature.