$ sudo apt-get update
$ sudo apt-get install firefox-mozsymbols 

firefox-mozsymbols is installed in your system.

Make ensure the firefox-mozsymbols package were installed using the commands given below,

$ sudo dpkg-query -l | grep firefox-mozsymbols *

You will get with firefox-mozsymbols package name, version, architecture and description in a table

firefox-mozsymbols package basic information: