Is this a recent change in the Beta version you use (70.0b10) or did this happened before ?

Do you have a Drag space feature and a Title bar feature on the Customize palette page that you can toggle to see if this has effect?

hi @cor-el, thanks for the advice!

I tried toggle the title bar and while it didn't actually "solve" this issue, I realized it was not an issue with Firefox per se. Since I was focusing on that one-pixel gap I noticed standard Mac apps with the system title bar also did not fill the gap completely.

I think it's simply the pure black border of Firefox made me notice that gap. (And now I can't un-notice it, :S)

OK I think I finally figured out *what* is bothering me:

Attached two screenshots of the "Light" and "Dark" built-in themes of Firefox. I'm using the ColorMeter to zoom in on the pixels.

In the "aperture" window, the top black part is the menu bar. Then there's a thin line of yellow which is my background. (Which I discovered that other Mac apps' title bars are also rendered this way, with a gap)

However, what makes Firefox stand out and caught my eye is that for the Dark theme, while the window itself is rendered black, the very top border of Firefox window is rendered in a much lighter grey color.

In the Light theme, the top border (of Firefox window) is also a different shade of grey than the rest, but it's a much subtle difference.

So the core of the problem is why Firefox render the top border in a very contrasting fashion. (I tried some custom themes and my observation is that Firefox tries to "fade-out" the main color but it seems that it does a much better job when the main theme color is lighter instead of darker)