Please post an image which clearly shows this weird blank rectangle and it's location on your screen.

I'm not sure, but it sounds like the area where you would normally get email notifications pop up.

Here you go

Check to see what you have got in this location:

• 'Menu icon (3 lines)' > 'Options' > 'Options' > 'General'

'When new messages arrive' section.

• Select 'Show an alert' - I believe this is already selected.
• 'Select 'Show a tray icon' - I believe this is already selected.
• click on 'Customise' button

It sounds as if Thunderbird is starting up and downloading some new mail. Therefore you get the alert pop up. But as the alert is empty, it sounds as if you have not selected what should be showing in that alert pop up box.. Please check this and report back on results.

Select all three checkboxes

• 'Message Preview Text, 'Subject', 'Sender'
• Select the number of seconds eg: 10
• click on 'OK'

• click on 'OK' to save and exit Options.

Thanks for your responses

The values appear as you say; for both sections are set by default, I have not changed anything

Thunderbird on my server remains constantly switched on; the rectangle appears later on, never on startup of Thunderbird

Do you have new incoming emails processed by Message Filters which moved emails to different folder?

Under 'Tools' > 'Developer tools' select 'Error console' Please clear this. Then send yourself an email and immediately exit Thunderbird.

Restart thunderbird. When you see the empty notification appear, check the 'error console'. Create an image of the contents.

Then test to see if this occurs in Thunderbird Safe Mode. 'Help' > 'Restart with addons disabled'

If you still have the issue even in TB 'Safe Mode'.... I've located some bug reports which sounds very similar to your case.

• https://bugzilla.mozilla.org/show_bug.cgi?id=894198

a newer one:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1292963

Could you logon/create an account and go to the above bug report. Mention your OS, version of Thunderbird and state whether this has just manifested itself or whether it was experienced in earlier versions. State that it also occurs in Thunderbird Safe Mode. Attach the image of error console contents. - note you can find the 'Attach File' option above the comments at the link.

If you do have Message Filters operating, it would be helpful if you can perform tests to determine which (if any) filter is causing the issue. This would mean disabling one filter at a time and seeing whether the blank notification pop up box appears. If it stops appearing after disabling a particular filter. Please take an image of the filter and access that bug report to inform them of your findings and attach the image as before.

Many thanks for your assistance in this.