title: Can't get sourcemaps to work in Firefox
body: <p>For the upcoming version of the <a href="http://www.transcrypt.org" rel="nofollow noreferrer">Transcrypt</a> Python to JS compiler, support for ES6 modules was added.
  However I can't get sourcemaps to work in Firefox, while in Chrome they work flawlessly.
  I am using Firefox Developer Edition 59.0b13 (64-bit)
  My problem is that the (Python) sourcefiles aren't shown in the Sources pane of the debugger.
  The following files are all in the same directory and served by the node.js http-server:</p>
  
  <p>Python source:</p>
  
  <pre><code>from itertools import chain
  
  class SolarSystem:
      planets = [list (chain (planet, (index + 1,))) for index, planet in enumerate ((
          ('Mercury', 'hot', 2240),
          ('Venus', 'sulphurous', 6052),
          ('Earth', 'fertile', 6378),
          ('Mars', 'reddish', 3397),
          ('Jupiter', 'stormy', 71492),
          ('Saturn', 'ringed', 60268),
          ('Uranus', 'cold', 25559),
          ('Neptune', 'very cold', 24766) 
      ))]
  
      lines = (
          '{} is a {} planet',
          'The radius of {} is {} km',
          '{} is planet nr. {} counting from the sun'
      )
  
      def __init__ (self):
          self.lineIndex = 0
  
      def greet (self):
          self.planet = self.planets [int (Math.random () * len (self.planets))]
          document.getElementById ('greet') .innerHTML = 'Hello {}'.format (self.planet [0])
          self.explain ()
  
      def explain (self):
          document.getElementById ('explain').innerHTML = (
              self.lines [self.lineIndex] .format (self.planet [0], self.planet [self.lineIndex + 1])
          )
          self.lineIndex = (self.lineIndex + 1) % 3
  
  solarSystem = SolarSystem ()
  </code></pre>
  
  <p>Transpiled to js:</p>
  
  <pre><code>// Transcrypt'ed from Python, 2018-03-30 16:23:19
  import {__envir__, __nest__, __init__, __get__, __getcm__, __getsm__, py_metatype, object, __class__, __pragma__, __call__, __kwargtrans__, __globals__, __super__, property, __setProperty__, assert, __merge__, dir, setattr, getattr, hasattr, delattr, __in__, __specialattrib__, len, __i__, __k__, __t__, float, int, bool, py_typeof, issubclass, isinstance, callable, repr, chr, ord, max, min, round, __jsUsePyNext__, __pyUseJsNext__, py_iter, py_next, __PyIterator__, __JsIterator__, py_reversed, zip, range, any, all, sum, enumerate, copy, deepcopy, list, tuple, set, bytearray, bytes, str, dict, __jsmod__, __mod__, __pow__, __neg__, __matmul__, __mul__, __truediv__, __floordiv__, __add__, __sub__, __lshift__, __rshift__, __or__, __xor__, __and__, __eq__, __ne__, __lt__, __le__, __gt__, __ge__, __imatmul__, __ipow__, __ijsmod__, __imod__, __imul__, __idiv__, __iadd__, __isub__, __ilshift__, __irshift__, __ior__, __ixor__, __iand__, __getitem__, __setitem__, __getslice__, __setslice__, BaseException, Exception, IterableError, StopIteration, ValueError, KeyError, AssertionError, NotImplementedError, IndexError, AttributeError, py_TypeError, Warning, UserWarning, DeprecationWarning, RuntimeWarning, __sort__, sorted, map, filter, divmod, __Terminal__, __terminal__, print} from './org.transcrypt.__runtime__.js';
  var __name__ = '__main__';
  import {chain} from './itertools.js';
  
  export var SolarSystem =  __class__ ('SolarSystem', [object], {
      __module__: __name__,
      planets: (function () {
          var __accu0__ = [];
          for (var [index, planet] of enumerate (tuple ([tuple (['Mercury', 'hot', 2240]), tuple (['Venus', 'sulphurous', 6052]), tuple (['Earth', 'fertile', 6378]), tuple (['Mars', 'reddish', 3397]), tuple (['Jupiter', 'stormy', 71492]), tuple (['Saturn', 'ringed', 60268]), tuple (['Uranus', 'cold', 25559]), tuple (['Neptune', 'very cold', 24766])]))) {
              __accu0__.append (list (chain (planet, tuple ([index + 1]))));
          }
          return __accu0__;
      }) (),
      lines: tuple (['{} is a {} planet', 'The radius of {} is {} km', '{} is planet nr. {} counting from the sun']),
      get __init__ () {return __get__ (this, function (self) {
          self.lineIndex = 0;
      });},
      get greet () {return __get__ (this, function (self) {
          self.planet = self.planets [int (Math.random () * len (self.planets))];
          document.getElementById ('greet').innerHTML = 'Hello {}'.format (self.planet [0]);
          self.explain ();
      });},
      get explain () {return __get__ (this, function (self) {
          document.getElementById ('explain').innerHTML = self.lines [self.lineIndex].format (self.planet [0], self.planet [self.lineIndex + 1]);
          self.lineIndex = __mod__ (self.lineIndex + 1, 3);
      });}
  });
  export var solarSystem = SolarSystem ();
  
  //# sourceMappingURL=hello.map
  </code></pre>
  
  <p>And the map:</p>
  
  <pre><code>{
      "version": 3,
      "file": "hello.js",
      "sources": [
          "hello.py"
      ],
      "mappings": "AAAA;AAAA;AAAA;AAAA;AAEA;AAAA;AAAA;AAAA;AAAA;AACA;AAAA;AAAA;AAAA;AAAA;AAWA;AAMA;AACA;AAAA;AAEA;AACA;AACA;AACA;AAAA;AAEA;AAEA;AAEA;AAAA;AAAA;AAEA;AAAA"
  }
  </code></pre>
  
  <p>Anyone sees what's missing?
  The page looks alright in the Firefox.
  Only the maps don't function.</p>