Yes, well, the documentation tells me to go to Install Add-Ons which window does indeed invite me to install code for Firefox. Which, when I click on it, takes me to Mozilla’s page for the 3-year-old Firefox extension.
So far two people here have told me the answer is obvious and neither of their suggestions has yielded an answer. I don’t mind being told to RTFM if the FM actually has the answer! Not y experience here though.