[morsch]

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...

[1] https://wiki.mozilla.org/Platform/GFX/WebRender_Where

[barrowclift]

If nothing else, I really appreciate the summary, with all the comments and various threads on this it's a bit hard to follow exactly what the hold up is.

[negativegate]

It looks like you currently need to go to about:config and enable layout.css.backdrop-filter.enabled. It worked for me without enabling web render. Not that this does much good until it's on by default.