Can a law firm Garnish my MI income tax refund/credit? This is for credit card debt.

A garnishment requires a court order. A court order requires that a judgment be awarded. So, if a judgment was awarded, the plaintiff the judgment was awarded to may be able to obtain an order to attach your refund.

As Scott said, if they already have a judgement and the filed to garnish it, yes they may.

But collection agencies will normally lie all the time, I always advise never ever talk to them on the phone, demand everything in writing.

I was told by a bankruptcy lawyer last week that tax refund checks will be garnished for student loan defaults, back child support payments, unemployment overpayments, and back taxes owed... but credit card/collection companies should not be able to garnish federal tax refund checks even with a court judgement against you (state refunds may vary by state though)...
They can get a writ of garnishment on your bank account though so if you deposit your tax refund check, they can "freeze" any money in your bank acct over $300... the bank is supposed to not allow anything under $300; so try to cash any large check rather than have it sit in bank acct.