From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

ZER . A ‘fenced’ city of Naphtali (  Joshua 19:35 ). It follows Ziddim (properly Hazziddim [with art.]), which may be the modern Hattin , N.W. of Tiberias. The identity of Zer is quite uncertain.

Smith's Bible Dictionary [2]

Zer. (Flint). A fortified town in the allotment of Naphtali,  Joshua 19:35 only, probably in the neighborhood of the southwest side of the Lake of Gennesareth.

Fausset's Bible Dictionary [3]

A fortified town of Naphtali ( Joshua 19:35). From the names which succeed in the list Zer is supposed to be S.W. of the lake of Gennesareth.

Morrish Bible Dictionary [4]

Fortified city in Naphtali.  Joshua 19:35 . Not identified.

Holman Bible Dictionary [5]

 Joshua 19:35 Zer

International Standard Bible Encyclopedia [6]

zēr , zer ( צר , cēr  ; in Septuagint the verse (  Joshua 19:35 ) reads καὶ αἱ πόλεις τειχήρεις τῶν Τυρίων , ρ Ο2 κπ . ρ Ο2 τπ . ρ Ο2 λπ ., kaı́ hai póleis teichḗreis tṓn Turı́ōn , which implies a Hebrew text with הצּרים , ha - cūrı̄m , "Tyrians"; this must be an error): One of the fortified cities in Naphtali, named between Ziddim ( Haṭṭı̄n ) and Hammath ( el - Ḥammeh , South of Tiberias). If the text is correct, it must have lain on the slopes West of the Sea of Galilee. It is not identified.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [7]

(Heb. Tser, צֵר Rock; Sept. by misapprehension, Τύρος , Vulg. Ser), one of the fortified towns of Naphtali ( Joshua 19:35), where it is named between Ziddim and Hamnath; but from the absence of the copulative ("and") between this and the preceding name, as well as from the total ("nineteen cities") in  Joshua 19:33, it is evidently a part of the preceding name, Ziddim-zer. (See Ziddim). Schwarz remarks (Palest. page 182) that Zer is mentioned in the Jerusalem Talmud (Megillah, 1) as lying near Ziddim.