The installation system currently does not support retrieving firmware. This means that any network cards that use a driver that requires firmware to be loaded, is not supported by default.

If there is no other NIC you can use during the installation, it is still possible to install Debian GNU/Linux using a full CD-ROM or DVD image. Select the option to not configure a network and install using only the packages available from the CD/DVD. You can then install the driver and firmware you need after the installation is completed (after the reboot) and configure your network manually. Note that the firmware may be packaged separately from the driver and may not be available in the “main” section of the Debian GNU/Linux archive.

If the driver itself is supported, you may also be able to use the NIC during installation by copying the firmware from some medium to /usr/lib/hotplug/firmware. Don't forget to also copy the firmware to that location for the installed system before the reboot at the end of the installation.