Edit your ~/.bashrc :
#PATH=$PATH:$HOME/bin:/usr/local/bin
alias firefox="open -a Firefox"
alias l="ls -l"
alias la="ls -la"
and reload the .bashrc file using . ~/.bashrc or source ~/.bashrc
You might want to add the following line to your ~/.bash_profile in order to get the ~/.bashrc loaded at the start of an interactive bash session:
[[ -r ~/.bashrc ]] && . ~/.bashrc