A better user experience when a Firefox extension loads a new tab custom page
N.B. This workaround is for macOS, using Keyboard Maestro. I have not tried equivalents elsewhere. On Windows, AutoHotkey can bind a hotkey, send Ctrl+T, select and clear the address bar, then Escape. On Linux, AutoKey is the usual analogue; under Wayland you may need something like ydotool. The Firefox new-tab focus behaviour itself is not macOS-specific.
In the previous post I described Slinky, a self-hosted start page, and mentioned that I use the New Tab Override extension so Firefox opens it on every new tab. That works, but the default behaviour is a little ugly.