.the project is a community effort to develop the SeaMonkey all-in-one internet application suite
So, I would say, it is going to be supported.
In what form, I am not sure. There has been minimal effort in the development of Seamonkey2.0, which has been stuck at the same alpha1 pre for over a year, I believe.
I like the idea of 2.0, because it includes the Firefox add-on manager, and a few other Firefox goodies.
As security, I think that any OS should be safe enough, using safe browsing. I, never, had ME, and I know nothing about an automatic guard function.
Take a look at this page
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1
