Mozilla has begun to roll out a system addon that enables TLS 1.3 in Firefox. Based on bug reports, this addon was launched on April 3rd, 2017 and is being delivered as a silent update to Firefox that will enable TLS 1.3 for 50% of the users who install it.
System addons are silent feature updates to Firefox that Mozilla can push out as needed. In this case, the addon is called "TLS 1.3 gradual roll-out 8.0" and when installed changes the security.tls.version.max setting to have a value of 4. When this is done, TLS 1.3 becomes enabled in Firefox.
TLS 1.3 is the next major version of the Transport Layer Security (TLS) protocol and offer numerous advantages over the previous 1.2 standard. TLS 1.3 offers stronger encryption, faster negotiation, supports new features such as TLS False Start and Zero Round Trip Time (0-RTT), and includes protection against various attacks. You can read more about TLS 1.3 here.
You can check if you are using TLS 1.3 in Firefox by going into the about:config page and checking the value of the security.tls.version.max setting. If it is set to 4, then you are using TLS 1.3. If it is set to 3, then you are still using TLS 1.2.
In the example, below the security.tls.version.max setting is configured to 4, so TLS 1.3 is enabled.
If it has not been enabled, you can manually enable it by double-clicking on the security.tls.version.max setting and changing the value to 4 in the dialog box that will appear. Then click OK to save the changes..
Once TLS 1.3 is enabled your browser will be able to take advantage of the better security offered by this protocol.