I installed Exchange 2016 in a test environment and used a SSL cert from namecheap.com for securing OWA. Upon loading OWA in Firefox I got the following error:
Firefox will throw this error when its using an inferior encryption protocol such as SSL 3.0, TLS 1.0, TLS 1.1
I wanted to force the server and clients to use TLS 1.2 for best security. The following article below from Exchange Team Blog goes into more detail and the required changes to get it working. I did have to reboot the Exchange server after making the registry changes, an IIS reset was not enough.