I had this problem too...for me it was caused by theGamers Unite snagbar that I had installed. For some people it was yahoo. I found a solution here: https://support.mozilla.com/en-US/questions/879011#answer-250883 wherein I changed my computer's date to two days ago (before the certificate expired) opened up Firefox (with no issue!) and uninstalled the toolbar. I closed Firefox, updated the date back to the correct date, and opened Firefox. Seems to have worked, I haven't had the error window pop up since.

Hi Kmuzikar, thanks for getting back. I was a bit frustrated yesterday with this issue. Well, after doing some research last night, I had read in another post there being an issue with the date. I changed the date a few years but it still gave me the message. Then changed it back to the date and still got the message. Finally, ran a Malware scan on my system only to discover this morning I had no malware and my firefox is working again.

Still not quite sure what happened or where that message came from, but if changing the date yesterday was the solution, I guess that fixed it then.

Thanks again for your help.

I looks that the site has renewed its certificate (Valid from: 09/25/2011 00:00:00 GMT), so you should no longer see that error message.