I'm not seeing rounded corners on a PiP window (checked in DE/Beta and Nightly). If you see this on Mac then this is either a Mac feature or something that is new in Big Sur.

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

I've attached a screenshot as requested; PiP window is at the top-left.

I asked the question because I've seen other browsers implement the same feature, but without the extremely rounded corners that I'm seeing in Firefox (they're either squared or very slightly rounded off in those cases). I was hoping there might be a hidden option somewhere to adjust the window styling to use squared edges, or perhaps it could be be put into the backlog for a future release.

I notice that all windows that show in the screenshot (PiP, About Firefox, Firefox) have these rounded corners, so it looks that you use a theme that causes this or this is a new Big Sur convention if you see this on all windows.

Hi, you are correct in observing that MacOS Big Sur has a differently styled UI than previous versions of MacOS (theming the OS is not possible, from what I understand). However, as I mentioned in an earlier comment, other browsers have implemented PiP where the pop-out window has a less rounded window style than the main window; I've attached a couple of screenshots for reference (Apple Safari, Brave).

Perhaps this can be forwarded to a product manager for review?