At Microsoft's suggestion, ran the current release version of Firefox (98.0.1) using default settings, in a virtual machine running Windows 11 on the Developer Preview channel (version 22H2, OS build 22581.1). Launching Firefox within this environment produced expected results wrt the window corners. I've attached a screen grab of what I'm seeing in this environment and configuration.
Bug 1755119 Comment 15 Edit History
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At Microsoft's suggestion, ran the current release version of Firefox (98.0.2) using default settings, in a virtual machine running Windows 11 on the Developer Preview channel (version 22H2, OS build 22581.1). Launching Firefox within this environment produced expected results wrt the window corners. I've attached a screen grab of what I'm seeing in this environment and configuration.