This is related to #179. Current Firefox & Safari as well as IE 10 don't accept function declarations inside and UglifyJS 2.6.2 with compress options:

{
    "hoist_funs": false,
    loops: false,
    unused: false
}

sometimes applies such a treatment.

This caused syntax errors in mentioned browsers on current jQuery master (source from commit 3befe5911a) where we enabled strict mode. More specifically, this function in the built file is transformed to be contained in a negation of this if.