See 6.4. Loading Missing Firmware from the Debian GNU/Linux Installation Guide.
If a device driver requests firmware that is not available,
debian-installer will display a dialog offering to load the missing
firmware. If this option is selected, debian-installer will scan
available devices for either loose firmware files or packages
Which devices are scanned and which file systems are supported depends
on the architecture, the installation method and the stage of the
installation. Especially during the early stages of the installation,
loading the firmware is most likely to succeed from a FAT-formatted
floppy disk or USB stick. On i386 and amd64 firmware can also be
loaded from an MMC or SD card.
Official CD images do not include non-free firmware. The most common
method to load such firmware is from some removable medium such as a
USB stick. Alternatively, unofficial CD builds containing non-free
firmware can be found at
To prepare a USB stick (or other medium like a hard drive partition,
or floppy disk), the firmware files or packages must be placed in
either the root directory or a directory named /firmware of the file
system on the medium. The recommended file system to use is FAT as
that is most certain to be supported during the early stages of the
Have a good look at that section of the manual as it contains more information about installing packages and making sure they are updated correctly after install.
(Also, part 5.4. Troubleshooting the Installation Process suggests to switch to Virtual console 4 (VT4) to check for any specific error messages, and part 6.1.1. Using the graphical installer explains that Ctrl+Alt+Fx can be used to switch to VTx. But it does not help much in this case.)