A PRO TIP for everyone who has got a problem with downloading... You have to downgrade you Firefox version to "Firefox 58.0.2 (US English)", you can get it from their site: https://support.mozilla.org/en-US/kb/install-older-version-of-firefox
After downgrading, the plugin works perfectly! Good luck. This is the best downloader, with no doubts.
Keep it up!