I can reproduce (circle looks jaggy to my eye at least), on Ubuntu 24.04, with 100% pixel scaling (no HiDPI). I don't have any suggestions/insight offhand, but bug 1724057 does seem potentially-relevant, yeah.
Bug 1919439 Comment 5 Edit History
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I can reproduce (circle looks jaggy to my eye at least), on Ubuntu 24.04, with 100% pixel scaling (no HiDPI). I included the circular Firefox and Google logos in other tabs (in a background window) in my screenshot for reference. I don't have any suggestions/insight offhand, but bug 1724057 does seem potentially-relevant, yeah.