Well you wouldn't uninstall Safari. You'd just use the other browser (Firefox) and set it as default in Safari's preferences. That way any action that requires access to a web page (such as clicking on a web link in your e-mail program) will know to launch Firefox instead of Safari.
Safari will then remain, unused, on your system. Should you ever wish to switch back and make Safari your default browser again, launch Safari and head into its Preferences and pick "Safari" as your default browser once again.