That is a Windows setting. Check for Default Apps and make adjustment there.

I am having the same issue.

I have Thunderbird as my default email client. Though I do not have a Mail cpl in my Control panel since I uninstalled MS Outlook.

In settings there are four items

  1 .eml, which I set to Thunderbird 
  2 MAILTO, which I set to Thunderbird
  3 MS-UNISTORE-EMAIL, which is set to the Outlook app. There is no option to choose Thunderbird. There is only an option to find something in the MS store.
  4 OUTLOOKMAIL, which is set to the Outlook app. There is no option to choose Thunderbird. There is only an option to find something in the MS store.

I also found information that states that there are entries in the prefs.js file that need to be removed. I found 3 files that are associated with Mozilla Thunderbird and all 3 do not have the entries the other article said to remove.

I also found an article saying to use the registry editor to remove an entry related to MS Outlook. I do not have that entry in my registry.

So I am really puzzled that this issue is affecting so many people and no one has an idea how to fix it.

I have Windows 11 Pro with all the latest updates. Running 23H2.

If the fix for this is to uninstall Thunderbird and reinstall it then please let me know how to save my messages etc. I do not want to lose my information.

Thanks