I think you are overwriting your websites-object in every loop with the following line:
websites = new List<string> {"https://mbasic.facebook.com/groups/"+groups[i]}.GetEnumerator();
That’s why it only prints the last website.
Have a look at How can I add an item to a IEnumerable collection?
Furthermore this could help you, too: IEnumerable vs. IEnumerator and implementation
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solved Launch all URLs in webbrowser c# 2 seconds to 2 seconds