See https://bugzilla.mozilla.org/show_bug.cgi?id=1735738 for reference.
r.open("GET", this.manifestFile, false);
      r.onreadystatechange = function () {
        if (r.readyState === 4) {
          self._log("done\n");
          self.manifest = JSON.parse(r.responseText);
          if (self.testFilter?.length || self.xfaOnly) {
            self.manifest = self.manifest.filter(function (item) {
              if (self.testFilter.includes(item.id)) {
                return true;
              }
              if (self.xfaOnly && item.enableXfa) {
                return true;
              }
              return false;
            });
          }
          self.currentTask = 0;
          self._nextTask();
        }
      };
      if (this.delay > 0) {
        this._log("\nDelaying for " + this.delay + " ms...\n");
      }
      
      
      setTimeout(function () {
        r.send(null);
      }, this.delay);
    },
    _nextTask() {