In following example: (function() { const c = 0; var c = 1; print(c); })(); FF 3RC1 throws an exception at 'var c = 1;': redeclaration of const c. KJS (and squirrelfish) silently ignores the redeclaration and prints out 0. I think FF's behavior is more desirable. 'var c = 1;' is likely a bug if a JS program, but KJS let the exception continue so it is very hard for the program to figure out what's wrong.
This is a duplicate of bug 14611. *** This bug has been marked as a duplicate of 14611 ***