I can't see salt tolerance as a holdover of ancient "amphibians", as modern amphibians are no more related to extinct marine or brackish creatures than we are. The most basal salamanders are largely associated with mountain streams (Cryptobranchidae, most genera of Hynobiidae), as are many of the most basal living frogs (Ascaphidae, Leiopelmatidae, Megophryidae). Salt tolerance is often associated with the most "advanced" species (Salamandra in Portugal, Salamandridae; Ambystoma in Mexico, Ambystomatidae; Bufo in Europe and Anaxyrus in NA, Bufonidae; Fejervarya in Asia, Dicroglossidae; Platymantis in Melanesia/Micronesia, Ceratobatrachidae).
It's notable that in Ambystoma, a single highly complex clade adapted to arid environments has developed mountain stream, saline lake, and highly adaptable species. Likely the aquatic life phase creates aquatic opportunities like streams, while terrestrial adaptation to aridity creates pre-adaptation for saline tolerance. The related Dicamptodon is also stream-adated, at least in extant species. Extinct species of dicamptodontid were likely more variable in habitat.