Tte prove that you are human thing work with a mouse movement tracker, if you move the mouse to straightline or calculate movement to the checkbox, the algorritm supposed a bot. But normally this issue you have is due to some extension, browsersetting or security soft. In Vivaldi I checked it in a guest profile, this putt the browser to the default settings and without extensions. I think in Firefox there will be a similar methode to check it, otherwise desactive the extensions one by one and clear the cache.

I am not getting prompted any captcha when I visit the site.