Short answers, pulled from the story.
The Old Frisian period began in 1276 with the copying of the First Brokmer Codex and lasted until around 1550. Before this date, only about two dozen runic inscriptions from the 5th through 10th centuries survive to the modern day.
The earliest complete text found in 2015 is an interlinear gloss of the Psalms dated to approximately 1100. It sits inside a Latin manuscript and shows how Frisian was used alongside Latin for religious purposes.
Most existing documents are legal codices compiled into seventeen separate collections. One incunable contains Middle Low German and Latin supplements alongside the main text.
Old Ems Frisian became the ancestor of Saterland Frisian its only living descendant today. Other dialects like Old Weser Frisian developed into Wangerooge, Wursten, and Harlingerland Frisian all now extinct.
German linguist Theodor Siebs popularized the idea that English and Frisian shared a closer relationship than any other Germanic language. He coined the term Anglo-Frisian languages in his 1889 dissertation titled On the History of the Anglo-Frisian Languages.