In your toolbar find Tools, open this and click open Add-ons. There you will find Extensions, in there you will find E-mail notifier. Disable it or remove it.

Look, I'm just trying to cancel the darn thing and never have I spent so much time trying to locate a source that'll accomplish the deed! I've spent the better part of the last 30 minutes trying to follow your directions (and yes, I'm on the better side of 60, but I'm not too old to read instructions), but your information is lacking in simple wording. Please inform!

Do you have an Icon in the top left corner of the browser that says Firefox?