I recently downloaded the latest (v11.1) FreeBSD USB release installation image and burnt it to a USB stick. However, my Intel D945-GCLF2 based PC completely ignores booting from the USB stick when attached (but happily boots other USB sticks), and I suspect that it is because the USB image uses the GPT schema. I believe previous versions of FreeBSD (eg. 10.4) used to provide both MBR and GPT USB installation images so an MBR version is needed for v11.1 (and future versions) of FreeBSD.
As a test, can you grab the image from https://people.freebsd.org/~emaste/mini-image.amd64.xz, uncompress it and write it to a USB stick, and try booting in BIOS and UEFI modes?