The solution is to set browser.urlbar.resultMenu.keyboardAccessible to false on the about:config page.

When a user comments that keyboard use in Firefox is not as efficient as they’d like in terms of the number of key presses, it’s not a good idea to suggest something that is even less efficient because it requires moving a hand.

For some reason, the default keyboard command Alt+P, which is Option+P on macOS, did not work for me, but I remembered that there’s a way on about:addons to “manage extension shortcuts”, so I changed it to Shift+Control+P, and now it works.