Time to update the design Mozilla 🙂 please. at least give us an option in settings.
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Similar idea here: Improved firefox interface rounded corners and more modern
I think if you make boxes rounder add shadows too.
Maybe if Firefox existed in only the Windows environment than making it perfectly match the Windows UI but for those of us that use alternative OSes (be it the macOS users or Linux/BSD users like myself, often with Firefox being the default browser of our distro or the de facto standard), redesigning for the Windows UI would make it obtuse looking in our environments. Granted I do a lot of modifications to the way the OS displays windows (GTK themes and modifications to my window manager configuration) and to Firefox (userChrome.css) that already includes rounding corners (which should be possible with userChrome.css CSS) but this is not true of everyone. Windows may be 90% (or whatever) of the market but its not 100% thankfully. Mozilla should refrain from treating Windows as first class at the expense of other systems, which it has thus far. Let's hope that continues.