Re-enable test_preload_file_with_manual_data_download_pthreads under firefox. NFC (#19088)

Author: sbc100

test/manual_download_data.html

@@ -189,7 +189,7 @@
         Module['downloadedData'] = data;
         var jsDownload = download('manual_download_data.js').then(function(data) {
           console.log('downloaded js file');
-          Module['mainScriptUrlOrBlob'] = new Blob([data]);
+          Module['mainScriptUrlOrBlob'] = new Blob([data], { type: 'application/javascript' });
           addScriptToDom(data);
         });
       });

test/test_browser.py

@@ -422,7 +422,6 @@ def make_main_two_files(path1, path2, nonexistingpath):
     'pthreads': (['-pthread', '-sPROXY_TO_PTHREAD', '-sEXIT_RUNTIME'],),
   })
   @requires_threads
-  @no_firefox('https://github.com/emscripten-core/emscripten/issues/19080')
   def test_preload_file_with_manual_data_download(self, args):
     src = test_file('manual_download_data.cpp')