Weird! Have you checked the Roundness setting in the Properties panel (with a rectangle or the Rectangle tool selected)? If the units are set to %, any small amount of rounding will become more noticeable at larger sizes. (Likewise, a small pixel amount could become scaled upon transformation if the Scale Attributes option has been selected.) But it shouldn't start spontaneously rounding—which sounds like what you're describing. Are you at least able to adjust the roundness after the fact, using the Properties panel?
Well, After re-starting Fireworks and my computer, the issue seems to have gone away. Hopefully for good. I don't need that while designing concept website concepts.
Thanks groove25 for your input. Its much appreciated.