Login screen freezes in Firefox on Ubuntu after redirect to passthrough page

When trying to log into our account on the latest version of Firefox in Ubuntu 16.0.4, my browser freezes with a rotating circle and the message "Please give us a moment. We're almost there."  This does not occur in MacOS (with Firefox or with Safari).  

I've tried a number of methods of logging into our account.  One is from https://www.att.com/.  But when I start there, the page is missing the usual login widgets at the top of the page.  Login widgets are not missing from the Business page, only from the Consumer page.  

If I select "Pay my bill" at the bottom of the page, same thing.  If I select Manage my Account, I get a login screen, but then after it asks about billing options, it loads https://www.att.com/my/#/passthrough/.... and I get the same "waiting" screen.