As you may know, Firefox 52.9.0 ESR is the latest browser version that works with Windows XP and Server 2003 (maybe also Vista, but nobody uses Vista).
The Cloudflare index page had Sign Up and Sign In links, both redirected to correspondign forms. Later on, the 2 drop-down menus (Support & Log In) were created. Both menus do not work in this web browser properly. The only way to login was to visit directly the page https://dash.cloudflare.com/
Also, since about September 12th some changes were made in the Dashboard frontend. Both login and registrationf forms are not rendered, there is an animating circle on the white blank page and “p.addEventListener is not a function” error in the console.
The second issue is critical and need to be fixed since it prevents to login. First one is not critical since direct URI access allows to skip this frontend bug.
The corresponding ticket id is 2258626.
P.S. looks like Safari is also affected