Interesting-Strange? Audible Mail Notification

Interesting-Strange? Audible Mail Notification

Before migrating to Zoho Mail, is was using webmail provided by my ISP, which was on a Zimbra platform.  I accessed using Firefox browser which has an Add-On for Zimbra audible notifications.

Since moving to Zoho, I am continuing to use Firefox browser access and the Zimbra Add-On remains installed in Firefox.  Interestingly, I am receiving audible notifications when Zoho mail arrives, apparently through the Zimbra notifications add-on.

How is this possible?

Edit Update:  Apparently, this only works if the ISP Zimbra webmail is also open in another window of Firefox.