Firefox won't run on NVIDIA Shield K1

I'm using the Firefox browser (48) on my Mac and on the smartphones.

But it won't run on the Shield tablet. Oké, it will; but it's unbelievable slow.
You can't browse at a normal level with it.

The Shield has plenty of memory left, and everything else is fast.
All the mobile devices are one the latest Android and have the latest Firefox on it.

What's the problem here?

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