I have found something:
"(...) It is traditionally believed that the name Frank is derived from their usage of the javelin, which in Old Germanic is *frankon, the same case as the Saxons and their weapon of choice, the seax. The francisca is named after the Franks, because they used this weapon too.
"frank Look up frank at Dictionary.com
c.1300, from O.Fr. franc "free, sincere, genuine," from M.L. Franc "a freeman, a Frank," one of the Gmc. people that conquered Celtic Gaul from the Romans c.500 C.E. and called it France, from Frankish *Frank (cf. O.H.G. Franko, O.E. Franca). The connection is that only Franks, as the conquering class, had the status of freemen. Sense of "outspoken" first recorded in Eng. 1548 (frankly in this sense is from c.1540). The origin of the ethnic name is uncertain; it traditionally is said to be from the old Gmc. word *frankon "javelin, lance" (cf. O.E. franca; also Saxon, traditionally from root of O.E. seax "knife"), their preferred weapon, but the opposite may be the case. In the Levant, this was the name given to anyone of Western nationality (cf. Feringhee). Verbal sense of "to free a letter for carriage or an article for publication" (1708) is from Fr. affranchir, from the same source."http://www.etymonline.com/index.php?term=frank
It looks to me like a case of educated folk etymology appeared in English-speaking world, probably by parallelism with Saxon. On the other hand, perhaps *frankon was named after the tribe (just like the word francisca
) and not in the other way.