Keybindings are weird animals. I've always used Ctrl-L for that, which focuses the URL bar and selects it. I note that Alt-D will not remove focus from an existing text widget, so it doesn't work as reliably. Where does Alt-D come from? I guess it's a vestige of an earlier browser?

Also, as the OP is using a mouse action, it's worth pointing out that triple-clicking the URL does a select-all (by analogy to selecting the whole "line" in other apps).