A first mortgage or deed of trust can be superceeded by the tax lein that is placed on the property each year. All counties have ways for you to rememdy the situation if you get behind on the taxes before the courthouse step sale of the property for the back taxes. Many even have time periods where you can get the property back after it's been sold on the steps.
Also when the county sells on the steps the lender is typically the buyer to preserve the asset. They will pay hte taxes and get the property.