I don’t know if this is intended or a bug, but if you have installed an addon, then visit that page on AMO, the button still says + Add to Firefox.
Should it not say something else like Update Now? Or check and say Update Now if your version is out-of-date, then hide the button if the addon is up-to-date?
Also while the page is loading, the button is temporarily blue, and says Download Now, nut it changes to normal after the page has finished loading.