When I switched to Vista, one of the biggest annoyances was that Trillian started opening links in Internet Explorer instead of Firefox, even though Firefox is set as my default browser and works everywhere else.
Turns out it’s an extremely simple setting. Just open Trillian Preferences and click on Text Conversations.
In this dialog, uncheck the option for “Open links in a new browser window”
I have to assume that Trillian uses some sort of built-in “new browser” mechanism that isn’t compatible with Vista.