If you want to check if your SSL / TLS settings are correct, head over to https://www.howsmyssl.com/
To fix TLS in Firefox:
- In Firefox, type about:config in the address bar and proceed past the 'are you sure' warning
- Type security.tls.version.max
- Edit the value to 3
- Close the tab
See below image:
This was an instant fix for my issue with Asana, and maybe it can help you too.
Unless I'm the only Firefox user left...