Several non-standard Array functions (such as Array.concat) existed in MozJS-60 but were removed in MozJS-91. Note that the Mozilla issue report refers to these functions as "generic," but they would be best described as "static member functions" if we were to use C++ terminology.
https://bugzilla.mozilla.org/show_bug.cgi?id=1222547

Note that this does not apply to member functions like Array.prototype.concat, which are part of the standard and continue to work in MozJS-91.