Since this bug was only filed to track bugs that appeared when building with the macOS 11 SDK, I'm going to close it as fixed. We are not aware of any bugs that appear only when building with the macOS 11 SDK. The last bug of that type was bug 1672842, for which a fix landed in Firefox 84. I have filed bug 1696504 about using the macOS 11 SDK in CI.
Bug 1670751 Comment 6 Edit History
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Since this bug was only filed to track bugs that appeared when building with the macOS 11 SDK, I'm going to close it as fixed. We are not aware of any bugs that appear only when building with the macOS 11 SDK. The last bug of that type was bug 1672842, for which a fix landed in Firefox 84. I have filed bug 1696504 about using the macOS 11 SDK in CI and bug 1696513 about raising the minimum required SDK version for local builds.