Selection color doesn't work in Firefox

[deshack]

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[deshack]

Reopen, because selection color doesn't work when in the Customizer screen in Firefox.

[deshack]

Now both the default and the customized selection background work in Firefox too, but not in the Customizer screen.

I'm marking it as low just because for the majority of users it's not a huge problem, but the reality is you cannot preview custom selection colors, so I'll keep it open.