User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:83.0) Gecko/20100101 Firefox/83.0

Steps to reproduce:

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Actual results:

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Expected results:

Currently in Firefox, the corner radius of various pop-outs and menu items match the OS's appearance. For example, in Windows, the corner radius of the address bar is more boxy compared to the one on macOS, and same with the "Added to Library!" or "Sent!" (tab sending) confirmation boxes. This is because various elements of the OS such as the the window corners on Windows appear to be boxy and have no curves.

The same happens in macOS, as the corner radius of the address bar and pop-outs matches the radius of the window corners.

In macOS Big Sur, the window corners have a noticeably more round look. However, the curves of the window corners do not match the curves on the corners of the address bar and pop-outs in Firefox currently and should likely be updated to match the new look of the OS.

Attached is a modified version of Haik's file that was attached to bug 1663467 because it demonstrates what the corners look like on both macOS Catalina and Big Sur.

Here's another example.

So here's some information I found about this, if it helps. On the Photon Design System website, it states that on macOS and Linux, the borders of these so-called "doorhangers" (what I've referred to as popouts) are curved with a 2px radius, and Windows corners are pointy. After a lot of searching, I found this old bug for making these corners curved on macOS which seems to be related. (I tried fixing this myself, but that patch seemed far too complicated for me, so maybe someone who is more comfortable with how this works could step in if they are interested?) :)

Closing this bug because the Proton redesign fixes these issues.