Is TB set as the default email program under (Windows) Settings/System/Default apps?

That setting does not seem to matter. It was not set as the default program. But even after I set it as the default - this box continues to appear on program startup.

First, I would run TB under a Windows administrator account and see if you can get the setting to stick (for all admin. and limited user accounts).

If that doesn't work, I found some info that appears to have worked for other users:

https://bugzilla.mozilla.org/show_bug.cgi?id=314581#c17

but the registry entries don't appear here in Win10.

Another step that may help is to open Settings/System/Default apps/Choose default apps by protocol and set the MAILTO protocol to TB (do this in an admin. account).

WIN10 does not show the Thunderbird Registry settings at all. MAILTO is set to Thunderbird URL.

I am the admin user on my machine already, fyi.

Why is it that all the Registry keys are not getting set up with the Thunderbird version? I do not find any of the keys mentioned above in the registry.

Is there any other solution to this issue?

Can't say why those registry keys aren't present in W10, but note that the bugzilla discussion predates W10.

I don't have a solution, but I would uninstall TB (won't affect the profile folder), reboot, then install TB and set as default email during setup (and also check it's default in Settings as above). Or, you could try setting another app as default, e.g. Mail app, reboot, then reset TB as default. All done as administrator.

There is also a preference under Tools/Options/Advanced/General/Config. editor, mail.shell.checkDefaultClient that should be set to false (double-click to toggle), click OK to save preference, restart TB. Try this first, as it is the simplest.

I did uninstall and re-install Thunderbird as Administrator. Set Thunderbird as Default Mail client.

No change in behavior though.

Do not know why that option keeps coming up each time Thunderbird is started, and is really painful to see.

I'm using Win 7 & Thunderbird 45.5.1 solved problem by:

Tools/Options/System Intergration - "Always check if Thunderbird is default mail client on startup"..

It's unchecked, put a check mark on it & the that box will Not appear on start up... That's it... Revers Psychology ...

BlkBart said

I'm using Win 7 & Thunderbird 45.5.1 solved problem by: Tools/Options/System Intergration - "Always check if Thunderbird is default mail client on startup".. It's unchecked, put a check mark on it & the that box will Not appear on start up... That's it... Revers Psychology ...

That's interesting. Another suggestion involves setting a couple of preferences in Config. editor. Bug 1314236 indicates that the 'twisted logic' you found is currently being fixed.

The fix going out in the next version. It might take 1-2 weeks to ship the new version 45.6.0