I like to open a new window (not in the same window) in JavaScript with window.open on rollover (without click event) and it gets blocked by popup blocker in IE and Firefox. Is there a workaround for this?
There's no way around it. You can't force browsers to do something they're not willing to do (by overt design!), and if you were to figure out a way it'd be treated like a severe bug and fixed as soon as possible.
Firefox can be reconfigured (by its user, not by a page or its code) to change the list of events that it considers "safe" for window opening, I think.