It's driving me nuts! It was working last night and this morning, but this afternoon, back to a revoked certificate error. The problem seems to be unique to Firefox; every other browser I've tried works.
I've had an online chat with TSP's host company, and they have concluded the issue can only be resolved by the company that issued the certificate (Starfield). I have contacted them, and await a response.
Don't worry, I've not forgotten you. It's just out of my hands at the moment.