X-notifier 3.3.3 and later
Before go further, let you look at
X-notifier => Options => Accounts => [pick one of your host/account]
you will see either
[ ] Include spam (that is equal to 0 : do not include)
or
[X] Include spam (that is equal to 2 : include to notification)
If you pick either Include spam Alert notify or not,
you do not need to type 'about:config'