Well, evidently they implemented backdrop-filter about 8 months ago, but only for the new WebRender stack. And apparently they don't intend to implement it in legacy stacks.
So I guess your feature is tied to the support for WebRender. And that is pretty much limited to some Windows environments, if this article[1] is up-to-date. Both of the Firefox installs I checked don't seem to use it, Linux/X11/Ubuntu and Android (where it's apparently tied to e10s aka Electrolysis, which I could not tell if they even intend to implement for Android any time soon).
Kind of a mess...