What are you expecting the Mozilla team to do about this? This is not a "bug" as the software is doing what it is supposed to.

File an issue with github / Microsoft to put a border around their favicon.ico so it shows up better when dark themes are enabled.
Make sure the screenshot is using Chromium Edge browser in dark mode :). MS browser on MS website... Developers are often big proponents of dark themes so it makes sense.

Another possibility is for the Mozilla and Google people to discuss a new standard for favicon.ico and possibly add a:
favicon.ico and optional favicon-dark.ico. If they are discussing improving the "standard" which isn't really a standard then Apple should be brought in but they, as expected, don't care about anybody else usually and do their own "standard".

Perhaps the fruit is ripe though (pun intended) as Microsoft and Apple now both have a dark mode in their OS (Linux has had often broken dark mode for a long time)

I can also confirm issue with spotify webplayer. Wrong accent favicon on dark theme. Opera can choose correct one. Vivaldi had same problem as FireFox but after my report they fixed it

Today Spotify has correct accent favicon on FireFox dark theme (green icon instead of black) but I didn't get any FF updates. So seems that it was fixed on spotify side, interesting