Open W10 Settings, Apps, Default apps, and check that TB is the default mail app, then click 'Set defaults by app' at the bottom, on the next page click Thunderbird, Manage, and make sure TB is selected for all items as in the attached picture.

wselml file type did not have Thunderbird associated with it, so I set that to Thunderbird too.

That still did not fix it.

Does it help if you open Tools/Options.Advanced/General/System Integration, click Check Now... and select TB as the default E-mail and click 'Set as Default'?

Edit: Are you doing this from an administrator account?

Thank you... but that doesn't help either...

I found a discussion of a similar problem that suggests it can be fixed by deleting the HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail registry folder and restarting the computer, then resetting the default email program as already explained. I would export the key or backup the registry in case it doesn't work.

I deleted the key and restarted... When I went to reset the default mail program to Thunderbird, it was ALREADY set.

Now when I try to "send" a photo from windows photo viewer , nothing happens... Don't even get the error message any more.

Is Thunderbird the program that actually opens when you use Windows Explorer? I was getting a similar message with Firefox each time it opened, I kept trying to set it to default from the dialog box and from system default apps. Research through Google caused me to check my installed programs, where I found two different entries for Firefox, one for 32-bit and the other for 64-bit. When I deleted the 32-bit version, I was never asked again about making Firefox the default. Thunderbird has not yet released a stable 64-bit version, but is it possible that you have two different versions of Thunderbird installed, but in two DIFFERENT file locations? This could easily happen if you 'custom' install and choose a different location from a previous version. The duplicate version, if it exists, may be the source of the 'default' dialog

When I click the 'E-mail' button in Windows Photo Viewer in W10, it attaches the image to a new TB message. If it's not working for you, the only other thing I can suggest is to run the fixmapi.exe utility. (See kb article).