You may not find a specific e-mail handler listed in Brave Browser settings. The solution is in the e-mail application’s settings:
Set Thunderbird as your Operating System’s default mail application.
Make sure, in those settings, to Enable Always check to see if Thunderbird is the default mail client on startup
IF, in a Brave Browser New Window at brave://settings/handlers you see anything like:
THEN delete/remove that/them.
Exit / Quit everything and Restart computer.
Ref. from June 2025: ‘Brave opens a browser instead of mail app when clicking on a mailto link’
More ref.:
‘https://support.mozilla.org/en-US/kb/make-thunderbird-default-mail-client?lang=en-US/’