In Firefox 12 I get in Firebug the error message :

Error: Showdown is not defined
(n)@http://2879097.run-a-gist.herokuapp.com/application.js:8
d((function ($http) {var showdown;showdown = new Showdown.converter;return {restrict: "E", compile: function (tElement, tAttrs) {tElement.html("<article class='markdown' ng-bind-html-unsafe='markdown(" + tAttrs.model + ")'></article>");return function (scope, element, attrs) {scope[tAttrs.model] = "";$http.get(attrs.src).success(function (data) {return scope[tAttrs.model] = data;});return scope.markdown = showdown.makeHtml;};}};}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:26
((function ($http) {var showdown;showdown = new Showdown.converter;return {restrict: "E", compile: function (tElement, tAttrs) {tElement.html("<article class='markdown' ng-bind-html-unsafe='markdown(" + tAttrs.model + ")'></article>");return function (scope, element, attrs) {scope[tAttrs.model] = "";$http.get(attrs.src).success(function (data) {return scope[tAttrs.model] = data;});return scope.markdown = showdown.makeHtml;};}};}),0,[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:35
l([object Array],(function (a) {"use strict";try {var f = b.invoke(a);if (P(f)) {f = {compile: A(f)};} else if (!f.compile && f.link) {f.compile = A(f.link);}f.priority = f.priority || 0;f.name = f.name || d;f.require = f.require || f.controller && f.name;f.restrict = f.restrict || "A";e.push(f);} catch (j) {c(j);}}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:6
([object Object],(function (a, c) {"use strict";b.error.apply(b, arguments);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:35
d((function (b, c) {"use strict";var e = [];l(a[d], function (a) {try {var f = b.invoke(a);if (P(f)) {f = {compile: A(f)};} else if (!f.compile && f.link) {f.compile = A(f.link);}f.priority = f.priority || 0;f.name = f.name || d;f.require = f.require || f.controller && f.name;f.restrict = f.restrict || "A";e.push(f);} catch (j) {c(j);}});return e;}),[object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:27
([object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:28
c("markdownDirective")@http://code.angularjs.org/angular-1.0.0rc10.min.js:25
z([object Array],"markdown","E",(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:43
G([object HTMLUnknownElement],[object Array],[object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
w([object Proxy],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
w([object Proxy],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
w([object Object],(void 0),[object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:38
o([object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:36
([object Object],(void 0))@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
((function () {"use strict";b.data("$injector", g);c(b)(a);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:83
((function () {"use strict";b.data("$injector", g);c(b)(a);}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:83
([object Object],o,[object Object])@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
d((function (a, c, g) {"use strict";a.$apply(function () {b.data("$injector", g);c(b)(a);});}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:26
mb([object Object],[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
dc([object HTMLDocument],mb)@http://code.angularjs.org/angular-1.0.0rc10.min.js:15
()@http://code.angularjs.org/angular-1.0.0rc10.min.js:155
a([object Event])@http://code.angularjs.org/angular-1.0.0rc10.min.js:111
(a,0,[object Array])@http://code.angularjs.org/angular-1.0.0rc10.min.js:22
l([object Array],(function (a) {"use strict";try {a.call(b, c);} catch (e) {}}))@http://code.angularjs.org/angular-1.0.0rc10.min.js:6
([object Event])@http://code.angularjs.org/angular-1.0.0rc10.min.js:22

http://code.angularjs.org/angular-1.0.0rc10.min.js
Line 58