"Open all browser file types in a single browser I select" is good enough for both the average user and the "technocratic elite" in basically all cases, and there are certainly zero cases where choosing every file association manually is better for someone who doesn't know what they're doing.

I don't understand your complaint about Firefox's dark mode; I've been using it for years and I don't remember any pure black elements. And as far as extensions, Firefox may be missing a couple minor APIs but it's the only browser that doesn't block the important ones (e.g. the ones uBlock Origin needs) so ad companies can make more money.