Using terminal and the open command I am trying to open a new browser instance followed by 2 tabs
Set an alias to the executable inside Firefox.app.

alias firefox_cli=/Applications/Firefox.app/Contents/MacOS/firefox
firefox_cli --new-tab "site1"
firefox_cli --new-tab "site2"


For more help, run

firefox_cli --help


To remove alias, 

unalias firefox_cli