You want to find unnecessary costs and create a budget. You cannot build a realistic budget without historical data. Importing 2025 gives you a full 12-month baseline to see exactly how much you spend on groceries, utilities, and those hidden subscriptions throughout the year.
This is the tricky part. In accounting software like Firefly III, the "Opening Balance" must be the amount of money that was in the account on the date the account was created/started. Since your import data starts on January 1st, 2025, your opening balance must be the actual money you had in the bank on the morning of January 1st, 2025. "But I don't know what I had in the bank a year ago!" That is completely fine. Here is the trick to solve it without digging through old statements:
The difference between the two numbers is exactly what your opening balance was!
Firefly says valid balance is -$500. Your real bank says +$2000. The difference is $2500.
Go back and edit the Opening Balance transaction to be that difference ($2500). Everything will then align perfectly.