The FW software is generally just enough to bring the card on-line to the chassis, so that the chassis sup's ROMMON, and/or the booting IOS can, if needed, update execution software on the cards. This is why the FW version, often, matters very little, and also why the SW versions should (I recall?) match.
Assuming you're read the release notes for the sup's ROMMON, often the updates are exotic and more, often than not, you really don't need to apply. This, too, because the IOS, has the execution software it needs to control installed hardware, regardless of the FW version on the card. (Have you ever noticed, new hardware, like line cards, will often note they need a minimum IOS version, not a minimum ROMMON version?)
