Udover tilgængeligheden for en enhedsdriver kræver noget udstyr også at såkaldt firmware eller microcode indlæses i enheden før den kan fungere. Dette er mest udbredt for netværksgrænsefladekort (specielt trådløse NIC'er), men for eksempel nogle USB-enheder og selv nogle harddisk-controllere kræver også firmware.

With many graphics cards, basic functionality is available without additional firmware, but the use of advanced features requires an appropriate firmware file to be installed in the system.

På mange ældre enheder, som kræver firmware for at fungere, blev firmwarefilen placeret permanent i en EEPROM/Flash-chip på selve enheden af producenten. I dag har de fleste nye enheder ikke firmwaren indlejret på den måde, så firmwarefilen skal overføres til enheden af værtsoperativsystemet hver gang systemet starter op.

In most cases firmware is non-free according to the criteria used by the Debian GNU/Linux project and thus cannot be included in the main distribution. If the device driver itself is included in the distribution and if Debian GNU/Linux legally can distribute the firmware, it will often be available as a separate package from the non-free-firmware section of the archive (prior to Debian GNU/Linux 12.0: from the non-free section).

However, this does not mean that such hardware cannot be used during installation. Starting with Debian GNU/Linux 12.0, following the 2022 General Resolution about non-free firmware, official installation images can include non-free firmware packages. By default, debian-installer will detect required firmware (based on kernel logs and modalias information), and install the relevant packages if they are found on an installation medium (e.g. on the netinst). The package manager gets automatically configured with the matching components so that those packages get security updates. This usually means that the non-free-firmware component gets enabled, in addition to main.

Users who wish to disable firmware lookup entirely can do so by setting the firmware=never boot parameter. It's an alias for the longer hw-detect/firmware-lookup=never form.

Unless firmware lookup is disabled entirely, debian-installer still supports loading firmware files or packages containing firmware from a removable medium, such as a USB stick. See Afsnit 6.4, “Indlæsning af manglende firmware” for detailed information on how to load firmware files or packages during the installation. Note that debian-installer is less likely to prompt for firmware files now that non-free firmware packages can be included on installation images.

Hvis debian-installer anmoder om en firmwarefil og du ikke har denne firmwarefil tilgængelig eller ikke ønsker at installere en ikkefri firmwarefil på dit system, så kan du fortsætte uden at indlæse firmwaren. Der er flere tilfælde hvor en driver anmoder om yderligere firmware fordi det skal bruges under bestemte omstændigheder, men enheden fungerer uden denne på de fleste systemer (dette sker f.eks. med bestemet netværkskort der bruger tg3-driveren).