Shirley, There were probably were no Ferstle's from Germany to the USA.The name is FERSTL. In Vienna there were some Ferstle's and one of them designed Weim, Vienna ring and the Viltakirche.Unfortunately he committed suicide at the ripe old age of 31.
What I surmise happened here in the USA after the first immigrants came over is that even today everyone added the English vowel, 'e' to the name and FERST is probably a derivitave of the FERSTL by dropping the 'l'. Those who added the vowel, 'e' did the same thing. I have the FERSTL Coat of Arms long before there was aUSA or the English Speaking Colonies.