I tried debian several times over the years, but it was with bookworm (debian 12) that i decided to make the switch on all my PCs and laptops, macbook included.
(mainly, it was the fact that the installer finally included firmwares out of the box which made installing much, much easier on laptops)
Because i want updated packages, the first thing i do is enable backports (otherwise i think that trixie still comes with kicad 5? hugh!) and do a full upgrade.
as for firefox, debian's repositories use firefox esr, which is why you are still on 128. There are instructions on firefox's site on how to switch to the regular release channels, just do that. If you can't trust firefox's own sources i don't know how you can trust debian's.
Debian + KDE is my favourite combo. I don't do anything different for desktop. When there was the debian 13 freeze i simply waited a couple of days, edited the sources to point at trixie and did a full-upgrade and an autoremove to clean old stuff. That's it.
How do you find the hardware compatibility to be? I've been keen to switch away from Ubuntu for years now and Debian would be my first choice but I'm wary of having problems e.g. with Nvidia GPUs, random peripheral devices such as printers and scanners, all of which mostly "just work" with Ubuntu. For this reason I'm leaning more towards Linux Mint but I'd like to be persuaded on Debian.
I don't consider outdated packages to be a problem on any distro because I just use Nix (which doesn't interfere with other package managers) whenever I want a more recent package.
Debian user here: no nvidia here, but the printer (networked), wifi and scanner just work. As does my bluetooth mouse,
I know that's not your point and I'm not saying this to cherry-pick your argument but in case that's particularly relevant to you, Debian Trixie ships with Kicad 9 : https://packages.debian.org/trixie/kicad. If you're stuck with an earlier version, maybe you have a dependency blocking your updates.
I’m a big fan of the Debian+Flatpak combo. Super stable base system, plus bleeding edge, usually dev maintained, GUI apps.
Kicad is easy to compile manually. The build process is surprisingly smooth for something so complex.