Superb plugin, beautiful effect, tunable at will ! Rated 5 out of 5 stars
VivaldiFox gives to Firefox the appearance of Vivaldi, with possibilities to keep Australis-rounded tabs or Vivaldi-squared ones, with changing color following the website (cameleon effect), and possibility to define your own custom theme very easily (only 4 colors to choose, and you're settled !)
Overall, it's very nice, giving Firefox a whole new look'n'feel, but still tunable if you don't like something in it. Must-try !