No, probably not Gecko. I've just opened snag.gy, done what it said and saw the success.
Having said that, it was quick and dirty as I did not open an account, so I was not logged in & etc.
But it did work, see sig for sordid details. I only have pop-ups and Flash blocked, and my default UA string is <Mozilla Gecko rv:38.0 K-Meleon 76.0 20160503>.
EDIT: update global UA to re