SPAM

I have used this great little add on for years.  I have noticed for a long time now that spam sent from Me in My spam folder is on the increase...

I have disabled the notifier for 2 weeks in office and at home, (I'm online 18 hrs a day on different machines) and seen a reduction of about 75%...

Did you really mean "spam sent from Me in My spam folder"???
Or did you mean TO you?  Otherwise, how would you collect the outbound spam in a folder?
You think X-Notifier is spoofing your email address and sending out spam with your account(s)?

Or if it's spam sent TO you, you think X-Notifier is some sort of spam magnet?

Just because you disabled it and spam decreased doesn't mean a direct causal link.
Spam is always a constant barrage, with ebb and flow.

And you're using X-Notifier in Firefox, not Chrome,  yes?