Firefox.

It's the only browser which can give me certain very simple behaviours. For example, noise the bookmark toolbar a lot. And I want bookmarks from that toolbar to automatically open in a new tab when I left-click on them. Firefox can give me that, whereas Chrome and most Chromium-based browsers don't even give me the option. I have to middle-click or ctrl-click, otherwise that bookmark mercilessly opens over my current tab. Could I teach myself a new habit (middle-click instead of left-click in the bookmark bar)? Sure. Should I have to?

The only Chromium browser that was able to give me this behaviour is Vivaldi, and that had a whole lot of other problems.

The other killer Firefox feature is tab containers. Wouldn't want to go another day without them.