With the patch from bug 1697331, and widget.macos.respect-system-appearance set to true, nested items in the bookmarks toolbar looks like in the attached screenshot in macOS Dark Mode.

The problems I see are:

• The background is too bright. Compare this to Safari in the next screenshot
• The arrow appears dark when it should be light

I realize that the "background being too bright" is invalid. I'll remove it from comment 0. The remaining problem is the black arrow.

Looks like this will get fixed as part of bug 1703051.

(In reply to Mike Conley (:mconley) (:⚙️) (Catching up on needinfos) from comment #3)

Looks like this will get fixed as part of bug 1703051.

Sort of. This will still be an issue in any other non-native but native-styled nested menus. The outstanding issue is the black arrow, which should be replaced with something that respects the system theme. I'll update the bug accordingly.

Where can I find "non-native but native-styled nested menus" that still show this black arrow?

The only similar arrows I could find was this: https://searchfox.org/mozilla-central/search?q=item-arrow&path=&case=false&regexp=false

Which is only used in Devtools in the Style Editor.

There's a couple in the dark mode Library window toolbarbuttons. Screenshot attached. This bug probably doesn't deserve to be a P2/S2, since these arrows are fairly rare now and a fix would be difficult. Afaik, these arrows are included in CoreUI under the names MenuSubmenu and MenuSubmenuLeft. We reference them here and they're drawn here. Since the icons aren't dark-mode aware, we'd have to refactor our non-native menu code to draw these arrows in a more modern way.

Markus, am I missing something here?

I think we could use [NSImage imageNamed:@"chevron.right"] for this, or imageNamed:NSImageNameRightFacingTriangleTemplate. Those special NSImages use dark-mode aware drawing. But we'd need to test what chevron.right draws on older macOS versions.
But given that these menus are now super rare in Firefox, and that bug 1710459 will at some point take care of them, I'm not sure if it's worth spending time on this bug.