Can you post a link to that website?

Then we can check which ciphers they support.

Did you disable any ciphers yourself?

You can check the ssl and tls prefs on the about:config page.

If any ssl or tls pref is bold (user set) then right-click that pref and choose "Reset" to reset the pref to the default value.

Use the Filter bar at the top with this regular expression:

• /security.*ssl|security.*tls/

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Thanks for the reply! I did not disable any ciphers. I checked the ssl and tls prefs and all were good. The site is really secure and unfortunatley I can not post it on the forums.

Did you check which cipher is used on the other computers?

Are the certificates up to date?

Can't you install the latest Firefox 3.6.17 version?

See also:

• How do I tell if my connection to a website is secure?