Warzone on Flash still works for me on Firefox, I just have to manually allow it everytime. In fact the main reason I use Firefox is because Flash doesn't work on other browsers (maybe it's just me, though). :P
How recently have you checked this, Onoma?
I manually allowed Flash but the 2020/01/12 time bomb goes off once I bypass Firefox's warning. I just get a clickable square of the Flash logo with an information icon, and if i click it, it takes me to the Flash End Of Life page.
Do you see something different, or are you just able to bypass that Adobe-level warning too?