That is a restricted Export Control, This means that it could potentially be exported to all countries except those that are restricted under U.S. or local law. It should be included in your download and install or available through your work if you are within a non-restricted country.

I need to know the ECCN code (like EAR99 or 5D992 or 5D002......)

thank you piero

Firefox and NSS are publicly available software not subject to the Export Administration Regulations (EAR) per EAR 734.3(b) and 734.7. Because Firefox is not subject to the EAR it does not have an Export Control Classification Number (ECCN). Mozilla has completed the notification for Firefox and NSS publicly available encryption source code per EAR 742.15(b). https://wiki.mozilla.org/Deployment:Deploying_Firefox#United_States_Export_Control_Information