It's unfortunate, but it's understandable if effort needs to be focused on a single good UI widget ecosystem fully under Mozilla's control, rather than living by the whims of the three major desktop UI toolkits they have to support, as well as the hundreds of thousands of web pages that are exclusively designed and tested against Chrome which already has been using non-native widgets across desktop platforms for a very long time. I'm not in the web dev space anymore, but I'd constantly see sites built that were incredibly dependent on the exact pixel sizes of widgets as they would render in Chrome, and would visually fall apart on Firefox, or with other zoom/text size settings.
UI design across Windows, macOS, and Linux GNOME/KDE have converged enough that it's probably good-enough if Firefox continues down the path of just theming their own widgets with the OS/user's color scheme where applicable, and calling it a day.
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