This comment is copy/pasted from a community issue filed during the Foxfooding program.

Firefox Version: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:90.0) Gecko/20100101 Firefox/90.0
Window Size (inner width and height): 1920x933

Steps to Reproduce

Firefox's 'System theme' looks like 'Light theme', without Windows accent color on the title bar

Expected Behavior

System theme should keep an OS accent color on the title bar

Actual Behavior

Firefox's 'System theme' looks like a 'Light theme'

Steps:

1. Choose "system theme" from Customize panel > Theme.
2. Set an accent color through the Windows 10 color preferences for interface elements. Check the checkbox that says "Title bars and window borders".
3. Close Firefox and reopen it again.

Expected result:
The Firefox title bar and tab strip follow the accent color set in WIndows' preferences.

Actual result:
The accent color isn't used at all.

As a user, I am confused as the theme is named "system theme", but only the dark/light settings in Windows are taken into account, not the accent color settings. Firefox, which is all about (easy) customization, is an exception as other applications do use the accent color in their title bars.

With the system System Theme it is really hard to see whether the window is active or not. The only indicator is the text in the tabs now which is switching between slightly different nuances of grey.

This is a major usability and accessibility problem, as evidenced by all the comments in the past week on this previous bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1701266

This really needs to get fixed before the browser.proton.enabled toggle is removed.

Ah yes, nice, marking it duplicate of a "won't fix" bug sends a clear message here:
We don't care what users think, it's their problem if they require a visibility setting that an almighty designer deemed useless.

Ah yes, nice, marking it duplicate of a "won't fix" bug sends a clear message here:
We don't care what users think

I do not work for Mozilla, so I do not send any "message". It is simply a duplicate and the decision won't be changed just because another ticket was opened. If you have new arguments feel free to post them in bug 1701266. There is no point in spreading comments about exactly the same issue across several tickets.

So how about this, Mozilla: If you abandon the Windows accent color and go with your own themes, fine. But please please please improve your Firefox dark and light themes in such a way that active windows can clearly be distinguished from inactive windows. This helps to avoid unwanted inputs into the wrong browser windows.

In defence of Mozilla, though, I have to say that many Microsoft apps that ship out of the box with Windows 10 don't respect accent color either: Weather, Microsoft Store and many others, especially the so called "Modern" or "UWP" apps, just to name a few. Other browsers like Opera, Brave and even Microsoft's own Edge come with their own themes that don't respect Windows accent color. No matter whether they are based on Chromium or other codebases. Mozilla now is in line with those other browsers. So complaints about this need to hit all those browsers and apps from other categories.
In my eyes this somehow indicates that Window accent color is not a main functionality / priority any more in Microsoft's own opinion specifically and in app development in general, otherwise they would maintain more consistency. So maybe users have to go with the flow (whether they like it or not) or pressure Microsoft to focus more on UI consistency (like Apple does it with their strict UI design guidelines).