Is it only with that one site, or are there others also?

Clear the Cache
Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

and

Remove Cookies
Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

If there is still a problem, Start Firefox in Safe Mode
While you are in safe mode;
Firefox Options > Advanced > General.
Look for and turn off Use Hardware Acceleration.
Poke around safe web sites and see if there is still a problem. Then restart.

I tried all that stuff and none of it helped. I can't load any of the google applications in firefox now. They pop up with this screen.

This Connection is Untrusted

You have asked Firefox to connect securely to mail.google.com, but we can't confirm that your connection is secure.

Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified. What Should I Do?

If you usually connect to this site without problems, this error could mean that someone is trying to impersonate the site, and you shouldn't continue.

mail.google.com uses an invalid security certificate. The certificate is not trusted because it was issued by an invalid CA certificate. (Error code: sec_error_inadequate_key_usage)

Now I tried a couple of other sites and they all seemed to work okay, and if necessary I could click understand the risks and still access the site, but no go with google. Considering I use my google account for my calendar and gmail a lot, I am highly disappointed with this new update. I hate using IE and really prefer to use Firefox.

Can you retrieve and view the certificate?

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"

Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".

• Click the "View..." button and inspect the certificate and check who is the issuer of the certificate.

You can see more Details like intermediate certificates that are used in the Details pane.

If "I Understand the Risks" is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".

• Note that some firewalls monitor (secure) connections and that programs like Sendori or FiddlerRoot can intercept connections and send their own certificate instead of the website's certificate.
• Note that it is not recommended to add a permanent exception in cases like this, so only use it to inspect the certificate.