However with Firefox beta, it does not work well. It is steady for sometime and then it crashes and then does not remember the login (although cookies are enabled) and reverts back to the QR code screen(I.e it forgets the login details) on web.whatsapp.com and again it has to be paired up with the phone. Why is this happening?

I have tried WhatsApp web both on firefox as well as Firefox beta. In Firefox it works like magic, remembers the login and works very well. Even if it crashes, it logs on the next time without any problem. However with Firefox beta, it does not work well. It is steady for sometime and then it crashes and then does not remember the login (although cookies are enabled) and reverts back to the QR code screen(I.e it forgets the login details) on web.whatsapp.com and again it has to be paired up with the phone. Why is this happening?