Firebrand is a stand-alone set in its own universe. Read it whenever.
The Requite books are a bit more complicated. The Hawkwood War follows directly after The Maker’s Mask and they started life as one book. If you want to read The Hawkwood War, you’re best off reading The Maker’s Mask first.
Heavy Ice is set two hundred and fifty years after the end of The Hawkwood War. You’ll probably catch more of the in-jokes and cultural references if you’ve read the first two Requite books, but it can stand on its own.