The icon is a green drop, you should see it.
If not, then you can click on the 'Show sidebars' icon

https://prod-cdn.sumo.mozilla.net/uploads/gallery/images/2017-11-03-00-22-36-0838fe.png

from Firefox in the top left conner, and in select Drop feeds in the sidebar list

Edit:
What is your configuration (OS, Firefox version, installed extensions, ....) ?
I will try to reproduce your issue (the missing icon).

Edit2:
In Firefox 56 I had disabled the icon, because features (open/close the sidebar) associate with it didn't work in Firefox 56. Then if you use Firefox 56 there is no icon.

Edit3:
For Firefox 56 users, in 0.7.2, I have just enabled the icon and it shows a message to explain how to process.