TL;DR
Bitcoin-only firmware leaves out support for other coins. Less code means fewer places for bugs and a smaller attack surface. For a bitcoin multisig there’s no downside, so pick it where it’s offered.
Why less code helps
Every supported coin adds code for addresses, transaction formats and signing. Each of those is a place where a bug could hide. Firmware that only does bitcoin has less to review and less that can go wrong. It also means fewer updates that have nothing to do with you.
Which devices offer it
- BitBox02 and BitBox02 Nova: sold as a separate Bitcoin-only edition.
- Trezor: Trezor Suite lets you install Bitcoin-only firmware on the device.
- Keystone 3 Pro: offers a Bitcoin-only firmware.
- Ledger: no separate firmware. You install only the Bitcoin app, but the operating system is the same for everyone.
Anything you lose?
Only support for other coins. If you also hold other coins, keep them on a separate single-sig device; multisig is awkward for most of them anyway.
Related: open vs closed source · reproducible builds