Firefox 52 ESR would probably be the safer alternative. It will receive security updates until June 26, 2018. And I honestly just can't imagine Waterfox or othe

Firefox 52 ESR would probably be the safer alternative. It will receive security updates until June 26, 2018. And I honestly just can't imagine Waterfox or other forks trying to keep XUL compatibility to stay secure long after that, as XUL extensions used to be a major maintenance burden for Mozilla and these forks have only a fraction of the size of Mozilla.