SyntaxError: missing = in const declaration Firefox 50
Yes, this appears to be a bug in Firefox. The spec allows the use of const:
IterationStatement: for(ForDeclaration in Expression) StatementForDeclaration: LetOrConst ForBindingForBinding: BindingIdentifier BindingPattern(truncated and simplified)
It seems Firefox is incorrectly interpreting ForDeclaration as a LexicalBinding.
Related: ECMAScript 2015: const in for loops
This seems like the bug report for this issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1101653 .
Proper let and const is coming to Firefox: https://twitter.com/evilpies/status/768881995912994816