Mark-to-Market (MTM) refers to the method of calculating values for positions based on daily movements of the position calculated against the closing or settlement price of the product for that day.  At the end of each business day, the open positions carried in an account are credited or debited funds based on the settlement price of the open positions that day. 

First In, First Out (FIFO) is the practice of using the first initiated position in a security as the trade that is paired off against the most recent closing trade in that same security.  This method is often used for tax accounting purposes.  In other words, it is the method of valuing securities which uses the oldest items in inventory first.