There is no reason for you enable FIPS mode. That is unless you're planning on doing a bunch of cryptographic work.
What's a FIPS?
The United States government defines many (several hundred) "Federal Information Processing Standard" (FIPS) documents. (FIPS sounds plural, but is singular; one FIPS document is a FIPS, not a FIP.) FIPS documents define rules, regulations, and standards for many aspects of handling of information by computers and by people. They apply to all US government employees and personnel, including soldiers in the armed forces, Generally speaking, any use of a computer by US government personnel must conform to all the relevant FIPS regulations. If you're a US government worker, and you want to use a Mozilla software product such as Firefox, or any product that uses NSS, you will want to use it in a way that is fully conformant with all the relevant FIPS regulations. Some other governments have also adopted many of the FIPS regulations, so their applicability is somewhat wider than just the US government's personnel.