If you are using older versions of Firefox or SeaMonkey, recently a security advisory
and could result in arbitrary code execution. ThunderBird is not affected, but Firefox 184.108.40.206 and earlier, and Seamonkey 1.0.7 and earlier are affected. This includes Firefox 1.5 branch from 220.127.116.11 and earlier.
If you're already using the latest versions, you aren't affected. If you aren't, it's a great reason to update