”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

hi, this has recently been an issue with avast/avg security software - are you using any of their products by chance?

The problem occurs after logging in to the Comcast/Xfinity web site. The next URL displayed is "https://customer.xfinity.com/#/", and then the Firefox error dialog ("A web page is slowing down your browser ...") is displayed. If I click on the Wait button the error dialog reoccurs almost immediately. If I select the Cancel button, then Firefox stalls -- cannot get to my "Home" page (which on my system is about:blank) by clicking on the Home icon. I have to kill Firefox (I use Microsoft's Process Explorer to kill Firefox),

I am using Firefox 59.0.2 (64-bit) on Windows 7 Ultimate 64-bit.

Internet Explorer 11 has no problem with this site.

I am not using Avast/avg security.