I have been using the CascadingDropDown extender control for several years now in an ASP.NET web site using version 15.1.4 The scenario in which the control is used is like select Make of Car in first dropdown, then select Model in the second. Just recently a Firefox user said the dropdown controls continually refresh several times a second as of about 22 Feb 2019 after Firefox updated to V65.0.1 They said it worked fine up until then. In fact he went to another PC with earlier version of Firefox and it worked, updated Firefox and the refreshing started. I just downloaded the latest version of AjaxControlToolkit (v18.1.1) on my development system and it does the same thing. I notice that while it is refreshing (several times a second) the IIS Express worker process is busy until the page is closed. Works fine in other browsers such as IE, Edge, Chrome, Safari.