with open("/usr/lib/firefox-esr/libxul.so", "rb") as f:
    data = bytearray(f.read())

patch = bytes([0x31, 0xC0, 0xC3])
offset = 0x443CAD0
data[offset : offset + len(patch)] = patch

with open("/usr/lib/firefox-esr/libxul.so", "wb") as f:
    f.write(data)