TLS 1.2 should be enabled already in Firefox without you having to do anything.

If having issues : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.3 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

There are TLS settings prefs on the about:config page that specify the minimum and maximum TLS version.

• security.tls.version.min = 1
• security.tls.version.max = 3
• 1 means TLS 1.0; 2 means TLS 1.1; 3 means TLS 1.2; 4 means TLS 1.3;
• http://kb.mozillazine.org/about:config

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

See also the security tab in the Network Monitor for more connection details.

• https://developer.mozilla.org/en/Tools/Network_Monitor

If necessary use "Ctrl+F5" or "Ctrl+Shift+R" to reload the page and bypass the cache to generate a fresh log.