@Andrew: Which version of live-build are you using? (lb --version)
We still use the version 20240810. We upgraded it from the bookworm version and it has been working fine till latest Trixie's changes (we build a custom ISO based on the mostly Bookworm packages + Trixie's kernel + debian-installer + some more not important updates).
I've checked the code and see it contains the "Firmware_List_From_Contents" function but a simpler version.
We will upgrade it after some tests. Right now we applied the attached patch and it works (but slow, of course).
Thank you all, guys. Now i see that we should use the latest live-build version (however, it might not work with bookworm packages, not sure right now).