Firmware is a kind of software, but one that you cannot interact with as you can for instance with an app, and it is always associated with some specific hardware – in this case it sits in your Base Station where it enables the Base Station to do its job. Disks have firmware that is loaded into a part of the disk to which you, the user, have no access, and which decides amongst other things where on the disk to store the items you save. A ROM contains a type of firmware, etc, etc.
Launch Airport Admin Utility (probably in your Utilities folder) and, now that you have Airport 4.2, you will probably be invited to download the firmware update for your Base Station.
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Andreas
G5 2.1GHz • Poking around in OS 10.5.4 • Working in OS 10.4.10