Updating your operating system doesn't usually cause issues to the way that content works in Firefox. Are you attempting to login to a web based email client? Also, are there any error messages that display when you attempt to login?

I use Frontier.com email. When I ask for it, Firefox replies that it can't establish a connection to the server at www.webmail.frontier.com. I have previously used Firefox with no problems.

I just tried visiting www.webmail.frontier.com and got the same issue. This must be because this address is no longer valid, as a result of the website being moved.

Try visiting login.frontier.com/webmail/ to login.