On the fibreglass I agree with the rest of them: don't do it, you'll be back! On the marble, you're better to take it out, check the underlying structure vis-a-vis deflection, thinset/screw some cbu down, then tile. Don't EVER use mastic in a situation where the tile might experience water. It turns into toothpaste which doesn't hold tiles down well :cheesygri Rich.
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