Can you please take a screen shot of that 'not secure' error and send it to us? You should be able to click on the broken padlock icon the top left of the screen to get some more details, and take a screen shot of that.
Hopefully we can get a better idea of the issue from that.
Mac El Capitan; Jaksta 5.1.1; Firefox 50.0.1
When Jaksta app is open and I open Firefox with Google as my home page, I am returned "Your connection is not secure" message. This did not happen before I upgraded Jaksta to 5.1.1
With Jaksta open and using Firefox I can access all other sites besides Google.