The original BSD advertising clause required that a notice go in all advertisements, which goes beyond trademark law. The license without that clause is compatible with pretty much everything (including the GPL).
Trademark law recognizes trademarks, and controls when they can be used. It doesn't put any restrictions on any activity that doesn't use somebody's trademark, such as legitimately using somebody else's code. The idea is that, when somebody uses FireFox, they can assume that this is something the Mozilla Foundation considers their browser. Stick to FireFox and you can rely on it as much as you can rely on Mozilla. You could regard it as formalized anti-fraud law, in that it prevents somebody from illicitly benefiting from or trashing somebody else's reputation.
The whole IceWeasel affair looked to me very reasonable in itself. Debian wanted a FireFox-based browser, and they got one. Mozilla didn't want to be directly associated with it after Debian did whatever to it, and they weren't. What's wrong with that? Other than all the ranting.
While trademarks are usually considered intellectual property, they are actually good for the general public. They don't stop anybody from doing anything other than misrepresenting the origin of something.