Let's try resetting the default browser one more time just to be certain the setting applied everywhere:

If you're using Windows:

   Set Edge as the default browser (again):

Go to Settings > Apps > Default apps Scoll to Microsoft Edge and click it Click "Set default" (this will apply to HTTP, HTTPS, and other web link types)

   Double-check individual protocols:

In the same screen, scroll down and make sure HTTP and HTTPS are both set to Microsoft Edge

   Restart Thunderbird

Close and reopen Thunderbird for it to detect the new system setting

 If you're on macOS like me:

   Change default browser:

 -     Open System Settings > Desktop & Dock

   -    Scroll down to Default Web Browser

   -    Select Microsoft Edge

  -  Restart Thunderbird

       Just like on Windows, Thunderbird uses the system default browser at launch, so restart it if it's still opening Chrome

let me know if this helped!

"In the same screen, scroll down and make sure HTTP and HTTPS are both set to Microsoft Edge" was the problem. Thanks!