Thanks for the suggestion!
In the meantime, you can use it as homepage if you choose custom url in option and paste the following url: moz-extension://5e20d7f7-341b-48dc-905d-cd0b53ecc4aa/moment.html

UPDATE: I have added a homepage option, but since FF doesn't support to set the homepage programmatically it is only the good link to use in custom url homepage.