That is a standard error message, but not particularly informative. It tends to indicate something is wrong with the computer at the website you are trying to use. Presumably the problem only occurs on one or two specific sites and not everytime you try to use firefox. * see also [http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_Server_Error (wikipedia)5xx_Server_Error] There is probably nothing you can do but it is worth clearing your browser cache, and any cookies for the site concerned. See * [[error loading web sites]] * & [[error code 500 accessing secure sites]]

Which security software (firewall) do you have? Some can cause such an error. You can also try to reset (power off/on) the router.