Okay...we are removing the old bent up roof vent frame, and it is adhered to the fiberglass. What can I use to break the adhesive without damaging the fiberglass?
I just replaced my old vent and had the same problem. There was butyl putty, silicone, and some other stuff (maybe bubble gum) under mine.
I found that a putty knife worked great. After you get it worked under the frame and lift it a little, you can work around the vent. Another possibility is to get a heat gun and apply some heat to it. That might soften up the adhesive. I'd tend to heat the metal frame and let the heat transfer to the adhesive, and start with a low heat and give it time to soften. Excess/misdirected heat could soften the fiberglass as well.
After I got the vent off I scraped it clean with a widget.