Speech and thought bubbles, respectively, are bubbles in which speech or thought is written in comics. I'm not sure but they might also be called speech and thought balloons but I can't remember. As for the boxes in comics, I have no idea what they are called, but I'm sure you could continue to call them "boxes" in lieu of a more appropriate label for them.
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Grammar GeekEDIT: Philip and I posted at the same time. I think cels are for animation, and each frame of the hand-drawn movie animation is a cel. I think for what we used to call "the Sunday funnies" they are frames?Curious!