Recently while trying to login to Dell OpenManage Server Administrator (OMSA), using the most recent Mozilla Firefox, I got the error pictured below. 
Initially I thought maybe the server process was hung so I restarted the service. That did not work so the next step was to try another browser. So I fired up Internet Explorer and logged in without a problem. So it has to be something with the browsers security so I headed over to trusty old DuckDuckGo and found the answer here. As it turns out, this error occurs on Firefox version 39 and above. As stated in the Dell Community Forums, the solution was simple. Login to OMSA using IE or Chrome, go to Preferences -> General Settings, set SSL Encryption to Auto Negotiate (pictured below highlighted in yellow)
and hit Apply. You’ll be notified to restart web server, click OK. Next open the terminal and restart the OMSA Web Server with the following command.
sudo service dsm_om_connsvc restart
Once the service has restarted, open up Firefox and it should be working as it had previously.