My Yahoo Mail Alerter is was working on June 26, 2019 in the morning, until that afternoon the icon is grayed out! I needed to bring back to fix the problem right away. I don't know the email is on my inbox or not, I am using Firefox 67.0.4 browser.

My Firefox Browser had updated into 69.0, and this note said this add-on gets blocked if you enabled tracker content blocking (Tracker or Third-party tracker cookies).