[https]
Type=Protocol
Handler=C:\Program Files\Mozilla Firefox\firefox.exe
Webhandler
Description
Flags=4

[http]
Type=Protocol
Handler=C:\Program Files\Mozilla Firefox\firefox.exe
Webhandler
Description
Flags=4

for example.

In operaprefs.ini, you'd have:

[File Types Section Info]
Version=3

[Trusted Protocols Section Info]
Version=1

However, if you try duplicating that for the standalone Opera Mail, it won't work as it doesn't make use of handlers.ini.

And, if you try adding:

[Trusted Protocols]
http="0,0,"C:\Program Files\Mozilla Firefox\firefox.exe""
https="0,0,"C:\Program Files\Mozilla Firefox\firefox.exe""

to operaprefs.ini like Opera did back in the day before things were moved to handlers.ini, it still won't work.

So, in short, you have to fix the http and https associations on Windows (in the registry if needed) so that Opera Mail is happy.

start "test" "http://example.com"

Then, try:

start "test" "https://example.com"

Do those open in the correct browser? If not, then that's why it's not working correctly in Opera Mail.