Firefox 43 supports TLS 1.0, 1.1, and 1.2 by default. You shouldn’t need to make any changes, but you can double-check the settings here if you like:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
(2) In the search box above the list, type or paste TLS and pause while the list is filtered
(3) If the security.tls.version.max preference is bolded and “user set” to a value other than 3, right-click > Reset the preference to restore the default value of 3
(4) If the security.tls.version.min preference is bolded and “user set” to a value other than 1, right-click > Reset the preference to restore the default value of 1
The values for these preferences mean:
1 => TLS 1.0 2 => TLS 1.1 3 => TLS 1.2