SeaMonkey is the last of the Community coded browser suites. The issue with it is that it is never ever signed (no one wants to put up the cash to get the credentials). Security Apps that look for valid digital signatures will always flag SeaMonkey as untrusted. As long as you do not install Nightlies and just stay with the verified releases you will be fine.
Very smart folk ensure the releases are malware free (we know what we are doing and prefer to keep this browser under the radar). Also no Blackhat in their Right Mind will ever target SeaMonkey as the game would not be worth the candle.