If you need this file, dump it from your own hardware. If you cannot, do not pirate it. Instead, use high-level emulators like Cxbx-Reloaded that do not require a BIOS at all. The cost of losing the entire Xbox emulation scene to legal action is far greater than the inconvenience of doing things properly.
XQEMU is a cycle-accurate emulator designed to mimic the Xbox hardware precisely. Unlike high-level emulators (like Cxbx-Reloaded), XQEMU needs real firmware dumps. A "portable" setup means: xbox bios mcpx10bin portable