I'm having the same problem with not being able to log in when you start using more features. I know this extension from chrome and it has a lot of potential once they fix their bugs in firefox.
Hope another update comes soon, Can't stand the dates starting on a sunday!
There have been a lot of changes/advancements in the past year for Firefox, I hope this is no longer the case for you. If it is, please file a bug report here: https://github.com/toolkit-for-ynab/toolkit-for-ynab/issues
