trouble running program in terminal after using 'su -l username'

[John Li]

I am having trouble running a program from the command line 
after I login as root and switch to a user account with su.

For example, after logging in as root, I want to get info off the 
web, but all my links are stored in my user account.  
So, I open a terminal, type
  su -l username
and then type 
  mozilla
Nothing happens, even when I specify the full path '/usr/bin/mozilla'.

Is there anything I can do to get around this?

John