Enter your long URL and click "Shorten" to get a shortened link which will automatically redirect to the long one. The number of requests you can make is limited to prevent spam. If you see a "Service Temporarily Unavailable" error, try again in a few seconds.
If you want to use this service in Firefox, you can install this add-on. It adds a "Copy ShortURL" entry to the right-click menu. You just have to set "Custom URL" to
https://0x.re/shorten?url=%URL%&pw=<password> in the add-on settings.