how do I know if I need to update my firefox browser

It will keep getting updates on its own and prompt you to restart to apply them in case there's some queued and you haven't updated for a while. You can also manually go to the top "Firefox" menu › first "About Firefox" item — and that dialog will show you your current version, start checking for any updates, and tell you if there's one ready with a button to restart to apply. Updates are usually rolled out over a few days across the globe, so not everyone gets the same version at the same time, but unless you're tracking down some specific issue, you probably don't need to bother with that. And of course you can always go to firefox.com to download the newest copy if you prefer to get the latest image yourself manually.