To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thank you for the helpful troubleshooting suggestion.

Disabling all Add-ons and Themes does not eliminate the issue. It occurs with the same frequency.

Oddly enough, https://bugzilla.mozilla.org/show_bug.cgi?id=1230628 - TB window focus blocked after click dismissing calendar reminder - reports a focus issue

Thank you. So it seems a similar bug process is happening with some other conditions. I'll keep looking for an answer and hope the next update will bring relief.

Just a quick update:

Add-on Disabled and Themes Disabled does not resolve issue. Recent windows 10 update did not resolve the issue. Uninstalling and reinstalling Thunderbird did not resolve the issue.

Issue is now happening sometimes after hitting 'write' to open a compose window. I can open a compose window, not click in it to type anything, and the focus will shift to the main window irregularly.

Does it also happen if you use the beta version from http://www.mozilla.org/en-US/thunderbird/channel/ ?

Does it happen also with version 38.5.0?

Thank you Wayne. I installed Beta version 43.0 yesterday and so far the focus window appears steady. I'll give it a few more days of testing.

I will also go back and try 38.5 and other versions, to see if a certain version provides relief, in case that's helpful info for development.

Evidently the stability was short-lived. The focus shifts targets during the 43.0 beta version as well.

Not sure where else to go with this. May need to look for another email solution.

Dropping in late to the party. But I think this post sheds some new light https://support.mozilla.org/en-US/questions/1106260

The person there has identified the issue occurs when the windows 10 wallpaper changes. Is that the case here?

Thank you so much Matt.

I read the other thread, disabled the slideshow background and am no longer experiencing the focus target shift.

Odd that this happens only with Thunderbird.

Is Bug 942310 this issue? I am not sure https://bugzilla.mozilla.org/show_bug.cgi?id=942610

But I think Bug 1234317 for Firefox certainly is. https://bugzilla.mozilla.org/show_bug.cgi?id=1234317

Looking through the linked comments for bug 942610, I see that the result of lost focus on the compose window is reported from various conditions. So the result is the same for a number of potential triggers. In that sense, bug 942610 may be this issue.

In my case, with Windows 10, disabling the slideshow background ended the loss of focus from the compose window.

I will play around with some of the other reported triggers to see if any helpful info surfaces.

https://bugzilla.mozilla.org/show_bug.cgi?id=1258152 covers the issue for Windows 10

Jhana, are you still seeing this when you use windows 10 and thunderbird 45?

Yes, the issue persists. With slideshow set as the desktop background, the Thunderbird focus window changes upon slideshow interval changes.

Only work-around I've found successful is to disable slideshow and go with a solid color.