Hover your cursor over the new tab button (at the top, next to the tab closing button). You will see the extension's unique ID, which will look something like this: 0f31e1c1-d63c-4660-b950-8bfbda3bb20a. Copy or note down this ID.

Open Firefox Settings and navigate to the Home section, or type about:preferences#home in the address bar.

Under the Homepage and new windows section, select Custom URLs...

Paste the following URL:
moz-extension://0f31e1c1-d63c-4660-b950-8bfbda3bb20a/index.html

(replace id with your actuall id)

This will set the extension as both your homepage and new tab page.