Are you using any add-on extensions with Firefox? Such as AdBlockPlus(ABP)?
I had an issue with CLEAR online account as it wouldn't display data usage stats on the home page. Turns out AdBlockPlus was blocking display. Disabling ABP on their page fixed the issue. CLEAR(Sprint) tech support didn't even know it could be an issue.
Firefox is my first choice of browsers. But it's usefulness by allowing add-on extensions can sometimes cause issues with particular websites.
You can easily test issues with extensions by re-starting Firefox in safe mode by clicking HELP > Restart with Add-ons Disabled.