Not sure if this is a browser issue or problem with TB. Saying you've done 'everything' doesn't help anyone trying to diagnose the issue. Check the mailto setting in Firefox Options/General/Applications, and the setting for mailto protocol in W10 Settings, Default apps. Does the File Explorer context menu Send to - Mail recipient work?

Thunderbird, not firefox.

Confirm this is set up. mailto setting in Firefox

• Menu app icon > Options > General
• scroll to 'Application'
• 'mailto' should say 'Use Thunderbird(default)'

Confirm this is set up. On Window 10 DEfault apps - Email is 'Thunderbird'

select 'Default applications by protocol' MAILTO should be Thunderbird.

Check Registry: In Windows search type 'regedit' and select 'Registry Editor' Locate this: HKEY_CLASSES_ROOT\Thunderbird.Url.mailto\shell\open\command after selecting 'command', the 'Data' column should say this....assuming Thunderbird is installed in default location on 'C' drive.

• "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" -osint -compose "%1"

Is Thunderbird installed in the default location or somewhere else?

Do you only have the one installation of Thunderbird?

For example: Have you installed Thunderbird on the 'D' drive and that is the one you use, but you still have another version of Thunderbird installed in the default location, but you do not use it?

Not using firefox.

Only one instance.

Again, to be clear, mailto links work, but unfortunately, thunderbird opens the "create newsgroups" dialog, rather than the "create email" dialog. And I don't have any newsgroups in thunderbird.

and all of the other registry entries appear to be correct.

"D:\data\PortableApps\ThunderbirdPortable\App\Thunderbird64\thunderbird.exe" -osint -compose "%1"

That string, above, (which is the data in the registry entry) when run from the command line, opens the "create news group" dialog, with an "email" set up behind the create newsgroup dialog TO THE newly created newsgroup.

Having now set up a newsgroup (I haven't used newsgroups since Bill Clinton was president), that string now opens a blank messsage to that new newsgroup.

"D:\data\PortableApps\ThunderbirdPortable\App\Thunderbird64\thunderbird.exe" -compose opens a blank message to a newsgroup

thunderbird.exe -compose "to='email@domain.com',subject='Some Subject',preselectid='id1',body='Message Body' opens a message to a newsgroup with the additional email in a to line, and the subject and the message body

After much experimentation, figured it out. This works correctly: "D:\data\PortableApps\ThunderbirdPortable\thunderbirdportable.exe" -compose "%1"

I thought the issue was likely to be due to a registry setting and/or a version of Thunderbird that was not installed in the default location. Many thanks for posting the registry information which you discovered works; I'm sure this may be helpful to others in same situation. I found the information you supplied to be valuable knowledge. The alteration required is probably due to you using PortableApps Thunderbird instead of the standard official version.