Free Software And Trademarks

Distributing a trojaned browser is illegal, but who will be able to sue the one who did it? The victims and the Government. Govenment is slowly taking up proscecuting computer crimes, but will they spend time on bringing such cases to justice? (Expect international criminal investigations.)
With the Firefox trademark protection the Mozilla foundation has a stick to hit the perpretrators.

Indeed. In legal terms, this is called standing. Standing is literally "what you stand on". If some one shot your mother, her death is your standing to bring charges against the killer. If you downloaded a trojan horse and lost all your money, your financial loss is your standing to bring the case to trial. If some one makes a trojan horse and puts your trademarked name on it, you can stand on trademark infringement and shut them down before your good name is damaged beyond repair.

When I was young my father taught me this much:

Son, the most valuable thing in the world is your good name. It's the hardest thing to get, and the easiest to use, so you protect it.

This is only them protecting their good name.

You obviously have a solid understanding of the fundamental purpose of trademark law and how controlling the Firefox trademark serves it. Why go into these creative ways that trademark can be exploited to solve problems it was never intended for?

There's an even more basic counterfeiting scenario than the ones you've proposed so far: A bunch of people, for whatever reason, start making their own variations on Firefox, under the same name. None of them is nefarious, but they don't work very well, and when they do, they all work differently. Now Firefox gets a bad name. People know Firefox as the thing that has more bugs than Internet Explorer and works differently on every computer. So they all cling to IE, losing the chance to use a superior browser: the original Firefox.