My sister seems to get your problem when she tries to login to school and they use out-dated browsers, try uninstalling Firefox then installing the latest version from
http://www.mozilla.com/en-US/firefox/ If this doesn't work
Make sure that you have the latest version of Adobe Flash Player installed.

If THAT doesn't work try this -
Is another BD account logged in on your computer while you're attempting to login to your account? If so, make sure that account is on world select when logging in, then you should be able to login.
Good luck getting it working again, an admin should be on to help in more depth soon.