From past experience, and the reason I browse with some fairly aggressive security settings (not the OP): I'm worried I could acquire a nasty rootkit-level virus.

I'm worried about that because I used to browse with no Noscript/similar, and then I got a infected by a rootkit, which I traced back to a new site I'd visited.

Since I didn't enjoy having to nuke my entire computer environment from orbit, I now have a rather locked-down browser.