How can I remove Firefox using terminal?
To remove Firefox.app:


Login with an administrator account
sudo rm -rf /Applications/Firefox.app/


You get the same result by just dragging the Firefox app from /Applications/ to the trash. 

If the crashes continue after reinstalling, deleting preferences and support files in the user library may help:


~/Library/Preferences/org.mozilla.firefox*
~/Library/Application Support/Firefox
~/Library/Application Support/Mozilla
~/Library/Mozilla
~/Library/Caches/Firefox