The "This page is asking you to confirm that you want to leave" notification is not displayed if the "New message" page is closed or refreshed

[Softvision-RemusDranca]

[Affected versions]:

• Firefox 63.0.3 and above

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile with the latest version of the "Email Tabs" add-on (0.1.8240) custom build.
• You are logged in your "Gmail Account".

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to a website (eg.:"https://www.wikipedia.org/".
2. Click the "Email-Tabs" toolbar button.
3. Click the "Email Tabs" button and select an email template.
4. Refresh the "New message" page and observe the behavior.

[Expected result]:

• The "This page is asking you to confirm that you want to leave" notification is displayed.

[Actual result]:

• The page is refreshed deleting the email's content and no notification is displayed.

[Notes]:

• This issue is also reproducible with the latest TP version of "Email Tabs".
• If the "Yahoo Mail" is set as email provider, the content of the page is not deleted after the "New message" is refreshed. However, the notification is not displayed if the page is closed.
• On Mac and Linux OS'es this issue is not reproducible on the first when you refresh or close the page. However on a second attempt the notification is no longer displayed.
• This issue is not reproducible with "Outlook" as email provider.
• Attached is a screen recording of the issue:

[wbt]

This appears to be an issue with Gmail; I'm getting the notice "This page is asking you to confirm that you want to leave - data you have entered may not be saved." when leaving Gmail even without "Email Tabs" installed.