using firefox, the popup with the sign in dialog never shows up. IE works, but it's buggy, looking awful, and with a lot of navigation not working correctly. on another PC with similar configuration, no problems. not sure the problems. both PC have the same version of windows and firefox with similar settings (Adblock+, private browsing, etc).
Are you using any plugins that block information being sent to the website to remain anonymous? Ghostery? Disconnect? ...or others? You will need to disable them to login initially, then you can re-enable afterwards. I know Ghostery blocks the login screen from appearing.
No problem with Win7 / Firefox on my end. But I can't speak to the login window popup, mine is set to auto-login.
There's an updated version of the theme that PriusChat runs that may fix that issue. Or it could be the AdBlocker. As someone who makes their living off of ad revenues, I'd appreciate you whitelisting priuschat.com from your AdBlocker, anyway.