I use NoScript, which is supposed to provide protection from "clickjacking", rogue js and XSS.
I have Firefox set to auto-delete everything, except the History (downloads and sites) and Session.
I also run Firefox using a vb script, which runs CCleaner when I quit.
Before I created my script and set the FF auto-cleanup, I had a mysterious hijacking incident (Yahoo Mail).
After multiple different AM & AV scans proved negative, I concluded that I must have forgot to manually clear my cache and something was lurking in there.
Since instituting my new procedure I haven't had any problems.