White border around window with dark theme

[CoryAlexMartin]

A white border appears around the window using the legacy theme with a dark theme selected in Firefox. The white border also persists in fullscreen videos.

Firefox 89

[sealvessel]

I also experience this. I used a pixel zooming application and I did not see any white lines near the edges in the light theme, so I am quite sure that the issue is that this is an issue with the dark theme.

[ideaweb]

Please could you checkit it with Firefox Nightly?

I ask because Firefox does not support real dark mode yet. A dark theme does not change the window itself to system dark mode currently.

But they made progress in recent Nightly.

https://bugzilla.mozilla.org/show_bug.cgi?id=1575070

Only its a bit strange that the white lines is visisble on all borders in your screenshot.