I held off on Windows 10 for as long as I could until Adobe, and therefore my job, required it. Now this nonsense. I hope this isn't the start of them joining on the web DRM bandwagon.
I just checked it out on mine. The regular version of Adobe Express works just fine in Firefox, but the "new Adobe Express (beta)" throws up an "unsupported browser" error in Firefox.
OP, look around for "use prior version" links on Adobe Express's website, as that will send you into the version that works in Firefox.