This is spun out of bug 1833390 to fix the Mac menu. See that bug and the Phabricator revision I'll attach for details.

Carmen, please see the screenshot in https://phabricator.services.mozilla.com/D178514#5889055 for what the non-native menu should look like now on Mac in dark themes. The menu arrow should be white and the separator should be lighter than before.

Edit: On 115, first you'll need to set widget.macos.native-context-menus to false to force Firefox to use the non-native menu, due to bug 1831760. If you're testing this using the weather menu, it will appear in the wrong place, too far from the Feedback button, but that's because 115 uses the native menu now which is opened using a different function. So ignore that part. On 114, once this is uplifted, Firefox will use the non-native menu and it should appear in the correct place with visible colors for the submenu arrow and menu separator.

Comment on attachment 9335071 [details]
Bug 1834135 - Improve the menuarrow and menu separator colors in non-native Mac menus in the dark appearance.

Beta/Release Uplift Approval Request

• User impact if declined: This bug is necessary for the Firefox Suggest weather suggestion feature we intend to ship in 114.
• Is this code covered by automated tests?: No
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: Please see comment 3
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): All this does is change the color of the submenu arrow and menu separator in non-native menus on Mac in dark themes so that they are more visible. This change isn't covered by an automated test because it's a styling change only.
• String changes made/needed:
• Is Android affected?: No

https://hg.mozilla.org/mozilla-central/rev/3a48e0599191

Comment on attachment 9335071 [details]
Bug 1834135 - Improve the menuarrow and menu separator colors in non-native Mac menus in the dark appearance.

Approved for 114 beta 8, thanks.

https://hg.mozilla.org/releases/mozilla-beta/rev/5e918c051f7b

I have verified this issue on the latest Firefox Nightly 115.0a1 (Build ID: 20230522094803) on macOS 12.6.1.

• The submenu arrow and the menu separator are both visible in the non-native macOS menu while having the Dark Theme enabled.

I have verified this issue on the latest Firefox Beta 114.0b8 (Build ID: 20230523183658) on macOS 12.6.1.

• The submenu arrow and the menu separator are both visible in the non-native macOS menu while having the Dark Theme enabled.
• Also, the menu is located just under the Feedback menu, not far from it.