Frimston is best remembered for his researches into Welsh Baptist history, e.g. Ebenezer: Hanes Eglwys Fedyddiedig, Llangefni, 1897; Canrif o Ymdrechion Bedyddwyr Môn, 1902; and several contributions to Baptist periodicals, especially on Flintshire Baptists in Y Greal, 1907-10, 1917, and on Anglesey Baptists in Seren Gomer, 1925-6. Other works by him were published, e.g. Y Cyssonydd Ysgrythyrol, 1885; Crefydd ym Mhlwyf Rhiwabon, 1890; Ofergoelion yr Hen Gymry (n.d.); and Holwyddoreg Ysgrythyrol (n.d.); and he was a contributor to Y Gwyddoniadur Cymreig. He also won two prizes for essays submitted to the national eisteddfod, at Rhyl (1892) on Flintshire antiquities, and at Pontypridd (1893) on Welsh emigration to America. He was secretary of the Books Society of the Welsh Baptists, and was president of the Baptist Associations of Anglesey (1897) and Denbigh, Flint and Merioneth (1926).