I am not confusing the 'software portal' with 'mintinstall'. They're one and the same
(more or less)
I don't use Opera so my answer may have been incomplete.
The Mint version of Firefox gives you the option "Install with" for mintInstall. That may, or may not, happen with Opera. Probably there are settings for default actions for different files in Opera.
But you can always download the small .mint file and double click it to start the install - you can do that in FF as well if you like