All my accounts disappeared after I restart Firefox
WebMail Notifier utilizes Password Manager of Firefox. If you clear "Saved Passwords", WebMail Notifier accounts will be gone. Don't clear "Saved Passwords".
You can set this option here.
Tools -> Options -> Private Data -> Always clear my private data when I close Firefox
Uncheck this option.
Autologin does not work
For autologin, you should check below option
Tools -> Options -> Privacy -> Accept third-party cookies
I want to check all unread mails in Gmail(including mails that are not in inbox).
Type about:config in location bar.
Find wm-notifier.webmail4.inboxOnly and set it false
How can I completely uninstall WebMail Notifier?
1. Uninstall WebMail Notifier in Firefox Add-on manager.
2. Delete preferences.
Type about:config in location bar.
Type wm-notifier in Filter.
For all the preferences that start with wm-notifier, right click and reset.
3. Delete accounts.
Go to Tools -> Options -> Security -> Saved Passwords.
Type wm-notifier in search.
Remove all in the list.