Edit. I meant to say function of your Internet browser.

Google mail to configuration for whatever browser you have.

I set TB as my Default Email Client in Win10 Systems and it worked for one day. This morning I am back where I was. I clicked on an icon to send a newspaper story via email, and Firefox opened the Gmail window again.

re :I clicked on an 'icon' to send a newspaper story via email,

I presume this 'icon' was on a website in browser 'Firefox'?

Sounds like you have an issue with one or more of the following:

1. the settings in your browser.
2. the settings of default programs to use MAILTO protocol.
3. I've seen instances where it could be the registry setting.

Check Mozilla Firefox browser settings:

• click on 'Open Menu'
• select 'Options'
• Select 'Applications'
• Set the 'MAILTO' protocol to use 'Thunderbird'

Double check your computer protocol 'MAILTO' setting is also 'Thunderbird'.

• Control Panel > Default Programs >Associate a file type or protocol with a program

Protocols:

• MAILTO needs to be set as Thunderbird.

Please check registry key:

• HKEY_CURRENT_USER\Software\Clients\Mail

• 'Start' > 'Run' > type: regedit

Locate this registry entry:

• HKEY_CURRENT_USER\Software\Clients\Mail
• on right side double click on 'Default'
• Enter 'Mozilla Thunderbird' in 'Value Data'
• click on 'OK'

Yup, that was it. Thanks, Toad-Hall (and OF COURSE it was Firefox--Mozilla is all I can trust!)