hi hoosiertech, since this is a fairly exotic combination, i'm not sure actually if it could work. judging by the specs of that device it is powered by an arm-cpu so i guess the system might be based on windows rt. firefox won't run on such devices: Will Firefox work on my mobile device? if your windows 8.1 embedded device comes with full support for regular x86 desktop apps somehow though, then you could use the regular download link for firefox on windows at https://www.mozilla.org/firefox/all/ - otherwise i fear you're out of luck...

What model is it as it probably uses a CPU that is not really compatible with the desktop Firefox on www.mozilla.org/firefox/all/

Newer ones seems to use a ARM type of cpu that is more typically found in a Android tablets.

The FZ-M1 which has Windows 8.1 64-bit has a i5-4302Y vPro processor which may be able to run desktop Firefox.

The newer FZ-F1 (with Win10 IoT Mobile Enterprise) has a Qualcomm MSM8974AB which is a ARMv7 cpu and same cpu used in FZ-N1 model which has Android 5.1.1.