get the location of UTXO data I need to find the location of the code which deals with UTXO, and then implement FIFO on UTXO storage. I aint able to find the location of UTXO code and also where it is stored.
What you want is the code for Florincoin's wallet's coin selection. This is found in the CreateTransaction function. Within CreateTransaction, the wallet also calls SelectCoins and that calls SelectCoinsMinConf. The coins are actually selected with SelectCoinsMinConf but SelectCoins and CreateTransaction are all involved with coin selection.

Also, because Florincoin's wallet is based on Bitcoin Core, it does not use FIFO for coin selection. Rather it uses Bitcoin Core's special algorithm for selecting coins which is difficult to understand (it tries to solve the knapsack problem by selecting exactly the amount you want to spend, but then it does some weird things to handle fees and change).