A squirrel found its way into my house. Is there any harm in allowing it to stay?

It found its way in through a screen with a hole in it that led into the attic and when I went up there to get fireworks it ran down the ladder into the house. Apart from the obvious messes it would have at first, do you think it could become accustomed to home living like a cat? Also should I get a litterbox? Would it know to use it? Do I need to buy squirrel food or can I just keep feeding it bird seed?