The dialog is an example of a dark pattern, with three options, one highlighted to encourage a reflex click, one in discouraging grey for customising settings, and sneaked in at top right, a small "Not now" link.If there's anything navigating "modern software" for a few years has taught me, it's that you should expect to be treated like the product and "herded", so the most encouraging option is usually the worst one for you, and the least encouraging option is usually the best. This anti-conditioning has become so strong for me that I remember recently being confused upon seeing a page with only a huge green Download button, thinking "this can't be the real one, it feels too fake and obvious" when it was actually the real one. I wonder how many others will be gradually conditioned by these dark patterns to do the exact opposite of their original sinister intentions.
But this Firefox one definitely takes the desire to confuse to new heights --- it's probably the first time I've seen "not now" as an opt-out, and I'd more instinctively go for the grey.