Ok, are we talking about running X but issuing command in a terminal, or are we talking about running at CLI level?
If you're running at CLI then X based applications simply won't run as they are designed for X. There are text based apps you can use. Links and Lynx are a couple of text based browsers. Links can be run with the -g option to give you a somewhat limited graphical browser display in CLI.
If you just want to issue commands in a terminal in order to learn the commands, then you need to learn what command starts what application. Most of them are found in /usr/bin and /usr/sbin.
Firefox can be launched by issuing the command firefox (kind of obvious huh?).
Manpages are your friend. If you need to know more about the syntax and options of a command issue 'man <command name>' and read the man pages for it.