firefox hangs when using fetch api

I have code like the following:
```
std::string buffer;
std::thread thd([&buffer, &filepath_absolute] {
emscripten_fetch_attr_t attr;
emscripten_fetch_attr_init(&attr);
strcpy(attr.requestMethod, "GET");
attr.attributes =
EMSCRIPTEN_FETCH_LOAD_TO_MEMORY |
EMSCRIPTEN_FETCH_SYNCHRONOUS |
EMSCRIPTEN_FETCH_REPLACE /* https://github.com/emscripten-core/emscripten/issues/8749 */
;
emscripten_fetch_t* fetch = emscripten_fetch(&attr, filepath_absolute.c_str()); // Blocks here until the operation is complete.
if (fetch->status == 200) {
//Corrade::Utility::Debug{} << "Finished downloading " << fetch->numBytes << " bytes from url: " << fetch->url;
// The data is now available at fetch->data[0] through fetch->data[fetch->numBytes-1];
}
else {
//Corrade::Utility::Debug{} << "Download failed. (url: '" << fetch->url << "', HTTP status code:" << fetch->status << ")";
}

buffer = std::string(fetch->numBytes, ' ');
memcpy(buffer.data(), fetch->data, fetch->numBytes);

emscripten_fetch_close(fetch);
});
thd.join();
```
On firefox, the call seems to just hang on the thd.join(). Things work fine on chrome.