Use WhatsApp desktop or web without a connection on the phone

On Android running Marshmallow, you'll want to disable doze for WhatsApp.

For stock Android, this link shows how to do that: http://www.greenbot.com/article/2993199/android/how-to-turn-off-doze-mode-for-specific-apps-in-android-marshmallow.html

On my LG phone, I went to settings -> General Tab at the top -> Battery & power saving -> Battery Usage

Scroll down till you see Battery Optimization and under that, tap 'Ignore optimizations'

Scroll down until you see WhatsApp and move the slider so it is on.

I'm using Waterfox 56.2.7.1 (64-bit) and all of the sudden, one morning, I started to receive the message below when I tried to access web.whatsapp.com:

WhatsApp works with Mozilla Firefox 30+ To use WhatsApp, update Firefox or use Google Chrome, Safari, Microsoft Edge or Opera.

In about:config privacy.resistFingerprinting was false. I change it to true and now I can access web.whatsapp.com, after pressing LEFT CTRL+F5.