Userscripts.org

Replace line 66 with:

  if (typeof GM_registerMenuCommand === "undefined") {
    // Running on GreaseKit on Safari, most likely
    // As such, need to specify options.path = /
    var options = new Object();
    options.path = '/';
    GM_setValue('rf_filters', json, options);
    delete options;
  } else {
    GM_setValue('rf_filters', json);
  }

Replace line 299 with:

  removeButton.setAttribute('class', 'btn');

(Using .class is generally frowned on, but in Safari it specifically breaks.)

I've made these changes to my version, works great (if you're also using the gm_functions script)