Theoraticly everything. But are they allowed, I think not.
Basicly they can program anything in their client, from keyloggers to screenshot making options.
Browser's tend to use the webpage title as their title bar, and FT Client can prolly check what programs are running and see their titles. Also browser's have options programmers can use to make plugins and such. For example, mIRC, an IRC client can also see what URL's ur browsing atm. So it's not that hard at all.