I am getting "This add-on requires a newer version of Firefox (at least version 55.0). You are using Firefox 52.0." But I am actually using Firefox 57.0! What sorcery is this?

Edit:

I downloaded version 1.1.4.2 of Stylus and was then able to update to version 1.5 from Firefox's add-on manager. Stylus add-on page still says my version of Firefox is 52.0, though.