hello immisi, ssl3.0 encrypted connections can be no longer considered secure since an attack vector against them ("POODLE") has become known. please contact the webmaster in charge of the inranet site and urge them to update their encryption to something contemporary. https://blog.mozilla.org/security/2014/10/14/the-poodle-attack-and-the-end-of-ssl-3-0/

if you want to re-enable ssl 3.0 in firefox you can do that - however keep the consequences in mind, that you will become vulnerable to the attack mentioned above all over the web: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named security.tls.version.min. double-click it, change its value to 0 and restart the browser.

You should reset security.tls.version.min back to the default 1 whenever possible as otherwise you may encounter another site you thought was secure but was not due to still using the vulnerable SSL 3.0.

You can also use this extension which will allow you to change it back and forth quicker. https://addons.mozilla.org/en-US/firefox/addon/ssl-version-control/