> Tampermonkey - Rating 4.7 (5,306 reviews) - 760,294 Users

> Violentmonkey - Rating 4.7 (793 reviews) - 164,622 Users

On Google Web Store, it's 11,000,000 users VS 900,000 users.

Their popularity seems different by a magnitude, hardly surprising Firefox/Mozilla would chose to focus on one above the other.