The grand old Frankfurt Opera House still stands, although it is now known as the Old Opera, having been replaced by a newer building. The original Opera House was inaugurated in 1880. Kaiser Wilhelm attended and remarked that he would not be allowed such an extravagance in Berlin.
At the time this card was sent (sometime before 1907), senders were not allowed to write a message on the back of the card. Many people scribbled message over or around the picture on the front. Salomon Marx simply imprinted the front of the card with his personalized stamp. He sent the card to Rachel and Esther Rousseau of Ghent, Belgium.
I couldn't find anything on the Rousseau sisters, but I did find that Salomon Marx was born into a prominent Jewish family in Frankfurt. I can only assume that this is the same Salomon Marx. He was born in 1863 and was able to emigrate (to the United States?) sometime after 1933.