I can access the uppcl.org site also and no problems and clicking the Online Bill Payment redirects me to the second link you gave without issue.
Note on my first connection to uppclonline.com, that also redirected me to the original image that I posted.
So I'm at a loss as to what is happening to you. As you say temporarily disabling the avast shields doesn't make any difference. So adding an exclusion is also unlikely to make any difference.
There is something else in the mix (other than avast, otherwise disabling it should have worked), but what that might be is the question.
What other security software do you have installed, antivirus, anti-malware, firewall ?
What security based add-ons do you have installed in firefox (NoScript, RequestPolicy, etc. etc.) ?