@jon@vivaldi.net Firefox because of its tweaks. I admire what Vivaldi has achieved, but Firefox can achieve almost the same with extensions. The only thing missing in Firefox are native tab groups, but they are a work in progress according to their site.
If Firefox vanished tomorrow, I would quite likely use Vivaldi. I was an Opera user in the old days. Vivaldi is the only browser that follows that spirit today.
@jon@vivaldi.net OK, I tried them. They are nice to have, but for users who manage many tabs at the same time. It is not my case.
One thing I like a lot about Firefox is the about:config page. My main change is content.notify.backoffcount > 0. It stops the browser partial refreshes, and shows a web page only when its parts finished loading. In my experience, it makes the browsing faster and more comfortable for my eyes.