I have some accounts that I need to be notified of messages much more timely than 15 minutes. Bluemail allows me to check "every few minutes" and it would be nice to have this option in Thunderbird as well.
Thunderbird already has this option, you just change the 15 to whatever number of minutes suits you.
You might violate your provider's terms, as some specifically preclude checking more frequently than 15 minutes. Outlooks answer is to refuse the login attempt.
Others may close your account. That is not something Thunderbird can control, however the option is there.