I think you are referring to a website and not an add-on. Does the site's sorting feature work in Firefox at all? If not, then perhaps they didn't test their code in Firefox. If it works but it's forgotten on different pages, there could be a cookie problem. The website may keep track of your sorting preference in a cookie so that as you move around the site, it has that information handy. Or maybe it is stored in your account if you are logged in, I don't know, I have never used it myself.