I've always seen it happen but have never really questioned it. I can see the torpedo's as having their own warp bubble which allows them to be fired in either direction during warp. The phasers are another in entirely, but I would assume that it also travels at FTL speeds which it makes it possible for them to be fired safely from a vessel during warp. Would the beams have to be at the same warp velocity to hit a target? or would they simply disperse once they enter the warp field of another vessel?
now I'm really curious..