[Linux] Make Firefox use its theme rather than GTK3 for "System theme — auto"

[cyangalaxy]

On every OS, except Linux, Firefox uses its own Dark/Light theme.

On Linux however, "System theme — Auto" uses GTK2/3 theme as the "Auto" theme.

You can see the difference for yourself, by switching "System theme — auto" <-> "Dark" / "Light" Theme.

[Jon]

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