Came across this obscure Firefox bug while building marshill.com/easter.

firefox_bug.js

/* In Firefox, all assertions pass except -8 and -9. Wat? Works fine in other browsers encountered */

function assert_equal(a, b) {
  if (a == b)
    console.log("TRUE")
  else
    console.log("FALSE")
}

assert_equal(parseInt("-01"), -1);
assert_equal(parseInt("-02"), -2);
assert_equal(parseInt("-03"), -3);
assert_equal(parseInt("-04"), -4);
assert_equal(parseInt("-05"), -5);
assert_equal(parseInt("-06"), -6);
assert_equal(parseInt("-07"), -7);
assert_equal(parseInt("-08"), -8);
assert_equal(parseInt("-09"), -9);
assert_equal(parseInt("-10"), -10);
assert_equal(parseInt("-11"), -11);
assert_equal(parseInt("-12"), -12);

assert_equal(parseInt("-01", 10), -1);
assert_equal(parseInt("-02", 10), -2);
assert_equal(parseInt("-03", 10), -3);
assert_equal(parseInt("-04", 10), -4);
assert_equal(parseInt("-05", 10), -5);
assert_equal(parseInt("-06", 10), -6);
assert_equal(parseInt("-07", 10), -7);
assert_equal(parseInt("-08", 10), -8);
assert_equal(parseInt("-09", 10), -9);
assert_equal(parseInt("-10", 10), -10);
assert_equal(parseInt("-11", 10), -11);
assert_equal(parseInt("-12", 10), -12);

UPDATE: easily resolvable using the second parameter to pass the base. Still weird that it's inconsistent across browsers.