hi, no firefox 50 is not a secure browser, since various security fixes have been release since then - always use the most current version of firefox (that's 53 at the moment)! Update Firefox to the latest release

Note that 128 bit encryption is very outdated. All Firefox versions since about 2003 (even before 1.0 got released) have supported 256 bit encryption, so you can ignore any such messages on a web page. Firefox used to include a 'N' field in the user agent, but this field has been removed a long time ago.

• https://en.wikipedia.org/wiki/User_agent#Encryption_strength_notations