User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:96.0) Gecko/20100101 Firefox/96.0

Steps to reproduce:

I set layout.css.backdrop-filter.enabled and gfx.webrender.all to true.

Actual results:

css backdrop filters aren't working on any website or even local files.
Tested on 96.0 and latest Nightly. all other (chromium) browsers on my system display backdrop-filter classes correctly.

specs.:
Operating System: KDE neon 5.23
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.3
Kernel Version: 5.11.0-25-generic (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-3770 CPU @ 3.40GHz
Memory: 27,1 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000