THIS website is actually using an older version, but I must say that I do not encounter any problems. This post is being written using Firefox 4.0.1 on Ubuntu (without any of the add-ons you mentioned in your post).
Perhaps one of these add-ons is the culprit. Could you try disabling each one, clearing your cache and retrying? If one of the add-ons is the culprit, please let us know so that we can investigate this further.
For best results, use the Firefox browser..