It seems almost all taxis in Frankfurt accept credit cards today. However, you should not expect that everywhere in Germany.
I would recommend to ask if credit cards are accepted, when you order a taxi by phone or just go to a free taxi. Note that there may be a small extra fee when paying by credit card.
About ATMs: While all ATMs accept credit cards, some charge you a fee, which can be 5% of your withdrawal or a minimum of 5€ (or more). For VISA, there is a web page where you can find the next ATM where you can get money for free, may be, there is something similar for Master Card.
Another point: Does your master cards have a golden chip? It's a security feature, and cards without it may not be accepted everywhere.
And as you may already have read: Credit cards are not very common in Germany. Hotels, gas stations and car rentals accept it, but that's it (mostly). So, prepare to have enough cash money to pay with you.