While my husband was busy scraping the old silicone caulk off our fiberglass tub, I suggested he try using nail polish remover, since I have used it to remove all kinds of sticky substances. Low and behold, it worked like a charm. We then went out to our local marine store and purchased a can of acetone.
Using cotton balls, clumped together, the old silicone caulk easily came off. First, apply generously with the cotton balls. Then, scrape with a putty knife or scraper. Came off in no time.