You can still plug in a FireWire PCIe card and have it work - I still use one for an old 35mm film scanner.

I think serial mice should still work as well - https://wiki.alpinelinux.org/wiki/Serial_mouse

Apple has finally depreciated Firewire with macOS 26, sadly.

Serial mice still also just work in Windows, too! If you attach a serial mouse to a USB-serial adapter, then attach the adapter, Windows should pick up and load sermouse.sys. On the flipside, if you’ve got a weird serial device attached screaming garbage out the wire, Windows might pick THAT up and load the mouse driver, too… “hey, why is my cursor freaking out?”