Firefox is telling me my connection is not secure when using the VIM Cloud. Here's how you can fix this...
Open a new window in Firefox and type "about:config" without the quotes, and hit enter.
Confirm that you want to continue
In the search field, type in security.enterprise_roots.enabled and hit enter - you'll be left with one field.
If it's marked as "false," double-clicking it should turn it into "true."
Close the window, go back to VIM Cloud Sign In and try again.
Source: Mozilla Support