web.whatsapp.com - desktop site instead of mobile site

[webcompat-bot]

URL: https://web.whatsapp.com/
Browser / Version: Firefox 54.0
Operating System: Windows 10
Problem type: Desktop site instead of mobile site

Steps to Reproduce

1. Navigate to: https://web.whatsapp.com/
2. Open any chat
3. Expand the Attachments menu.

Expected Behavior:
The attachments menu displays normally rolling down at a normal speed

Actual Behavior:
The attachments menu rolls down until Photos & Videos, then stops completely, then finishes rolling down.
Chatting after that becomes slow, the text input is delayed, and the search functionality is slow as well.

From webcompat.com with ❤️

[MDTsai]

Not able to reproduce using Firefox 54 on Linux.

[adamopenweb]

Let's close as duplicate of #7532.