Not great!
Instead of using an icon on the toolbar and having a settings menu that contains all the settings --- like normal extensions for Firefox for the past decade or so ---this app takes over your New Browser Tab -- which is where you actually control the rest of the settings.

I'm not comfortable having an extension that:
- Needs to have it's own tab to work.
- Takes over my new tab, where I keep important bookmarks.
- Does not have a toolbar icon. This seems suspect to me, why would you want me to forget that this extension is on my machine?
- Splits settings into two wildly different interfaces.



I can't use an extension with such poor UX.

When I open a new tab to go to an easy-to-be-distracted website, I see the Serene home tab layout which gently reminds me to get back to focus, especially if I am in a focus session when I wouldn't be able to use the easy-to-be-distracted website anyway. This extension is awesome.