Two things. First is that solvents won't dissolve silicone, but they can soften it by absorbing into it. Haven't seen it recently, but there used to be a silicone caulk specific solvent out there. I would try wetting the mess with toluene or xylene to first soften it. Wear gloves with that stuff, and don't do it indoors (but you knew that).
But even with it softened, you will still have to scrape it off. For that the best thing I know of is a flat razor blade (the single edge type) that has been dulled a bit with sandpaper, used almost flat against the surface. A fresh sharp blade will catch on the surface and scratch it, but if you blunt it a bit, especially at the corners, it will slide along the fiberglass surface without catching it, and allow you to lift off silicone or varnish or the like.