Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks for responding....but isn't your potential solution designed for Windows users? I have a Mac.

If it does apply to Mac, I am also hoping for something simpler as this could take weeks to isolate the problem and it would be more efficient to use another browser for mail

Also, I should have added, this problem occurs only occasionally and seemingly only when I am taking some time to compose a message

Some extensions or toolbars can add such an alert.
It can also be part of the code on a page to prevent dataloss if you've started to type text (compose a new mail) and you try to close that tab.
You should be able to close such an alert and close the current tab.

What I posted above applies to all platforms, including Mac.

perhaps I should be able to "close such an alert" but I am at a loss how since it keeps popping back up like a jack in the box