http://manezh.rossery.pp.ua/pape/register-dealer https://validator.w3.org/ 404 Not Found https://jigsaw.w3.org/css-validator/ not found so code is ok as can not scan it. Above are from the W3C.org who is in charge of web standards and practices, development and experimental code for Web Browsers and Web Pages. In other words follow the rules set by them for perfect code and google will love you. and https://validator.w3.org/i18n-checker/ and http://mobile.css-validator.org/ The debugger is not as strong as it was I here. Developers Edition is better so say, it updates weekly or so. so can be unstable. Can be run at the same time as 57+ as has a compatibility mode. So can refresh or Reinstall to try and fix the issue. uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/ Please let us know if this solved your issue or if need further assistance.