Tweaks

Putting these somewhere easy to find...

Firefox

Suppress the download list that automatically appears:

about:config → browser.download.alwaysOpenPanel set to false.

Compact mode:

about:config → browser.compactmode.show = true
☰ menu → More Tools → Customize toolbar ... → Density: Compact.

Preview

Suppress the Open File window that automatically opens:

System Preferences → Apple ID → iCloud → iCloud Drive [Options] → Uncheck Preview.