Thanks for your feedback!

Try setting it in your browser preferences:

1) To find the URL of the extension, open the extension's settings page (click the gear icon in the top-left corner). You’ll see the URL of the settings page(in the address bar), which will look something like this:
moz-extension://1f5e2951-cc18-45bc-ab08-e3d37f733710/options.html
Replace the options part of the URL with newtab, so it becomes:
moz-extension://your_extension_id/newtab.html

2) Then, type the following into the address bar:
about:preferences#home
There, you’ll find the field called "Home page and new windows".
Paste the modified URL from step one into that field - moz-extension://your_extension_id/newtab.html