If you're using Firefox 52 ESR, or a fork based on it, I've been releasing Flagfox 5.2.x ESR updates that support it. If not, please update your browser or switch to one of those; running a browser without its security updates is dangerous. You can download the latest Flagfox 5.2.x ESR version here:
https://flagfox.wordpress.com/2018/04/30/april-updates/
https://flagfox.wordpress.com/download/