this new version of XN is really not good. i've recreated all my accounts but only a few of them connect. most do not. when i click on the account it takes me to the right email system (gmail, yahoo, etc). but it won't log me in. i have to manually log in. i also cannot see if i have new messages in a folders other than my inbox. this version is not usable, IMO.
Changes...
Firefox is changing their software engine with versions 50+ ...this forces massive changes to X-Notifier.
X-Notifier 4.x is a work in progress, because FF 50+ is a work in progress.
(X-Notifier 4.x can no longer support POP3/IMAP or custom scripts.)
Firefox 49.x and X-Notifier 3.5.23 is the most stable and feature-rich combination.
For a time, X-N 3.5.23 can be used with FF 50+
To use X-N 4.x, you should first Export your accounts (backup) from 3.5.23, then update to latest X-N version 4.0.2, and Import your accounts backup (or just redefine all of them). Then Export them for a new backup.