Removing Dried Foam Insulation from a Fiberglass Bathtub

I assume you are talking about foam insulation from an areosol can. There stuff is a bear to clean up. has to be cleaned up as soon as it gets on a surface. I only have a few suggestions.
1. Find the manufacturer from the can and locate their address from either the can or the internet and ask them for help. They may know of a solvent that will disolve the form.
2. Scrape off the excess foam with a razor blade being careful not to scratch the fiber glass.
3. Use a non scratch cleaning pad with a solvent. Use ventalation with any solvent.
4. Do not use any abrasive material as that will scratch the fiberglass.
5. Contact a fiberglass refinisher and see if they have any answer. You can find them in the yellow pages or on the internet.