The paper does not recognize fluoride as a neurotoxin in its current application in Europe:
Overall, despite the remaining uncertainties, and based on the totality of evidence the present review does not support the presumption that fluoride should be considered as a human developmental neurotoxicant at current exposure levels in European countries.
Between these two options DuckDuckGo Browser is at least free and open source, while Vivaldi is closed source, which makes DuckDuckGo Browser the better choice.
Firefox and its forks are better than both. Firefox's Gecko engine is independent of Google and Apple, while Vivaldi uses Google's Blink engine and DuckDuckGo Browser uses either Blink or Apple's WebKit engine depending on platform.