If SeaMonkey comes up automatically it’s either because it has been set to start when your computer boots up, or it is your default email or browser.
1. Click on the Apple menu, then Accounts, and look at the login items for your account. Remove SeaMonkey if it’s there. Optionally you can add Safari to the list if you want it to come up when your computer starts.
2. Open Safari (if that’s what you want to use as your browser), then go to the Safari menu, then Preferences, and under General set Safari as your default web browser.
Does this answer your question(s)?