N. de Baumgarten in his "Genealogies et Mariages Occidentaux des Rurikides Russes du Xe au XIIIe siecle" (Roma, 1927) states on pp. 22-26 the details of the descendants of Great Prince Vsevolod I including Euphrosine. He lists her as the eldest child of Mstislav-Harald's second marriage in 1122 to the daughter of Dimitri Zawiditch, possadnik of Novgorod, which daughter died after 1168.
Some further details concerning this marriage can be found in Vol. XXV of the Camden Society's 3rd Series of publications, namely: "The Chronicle of Novgorod, 1016-1471" translated & edited by Robert Michell & Nevill Forbes (London, 1914). It is written that Mstislav married the still unnamed daughter sometime during 1122 in Kiev and that her father, Dmitri Zavidits, posadnik of Novgorod, had died July 9, 1118 (p. 10).
I suppose Dmitri Zavidich was probably either of Slavic or Scandinavian origin. Presumably he come from a family of Boyars in Novgorod as he was a Posadnik of that city.