Hello, you can get the latest version here: https://firefox.com

It is safe to install the new version over the old one.

If this solved your problem, please mark as solution.

And yes, you are two versions behind.

More like 7 versions if you include minor updates (57.0-57.0.4, 58.0, 58.0.1). Note that 58.0.2 is likely to be released during the next days.

If there a problem with updating or with the permissions then best is to download the full version and trash the currently installed Firefox application to do a clean reinstall.

Download a new copy of the Firefox application and save the Disk Image file to the desktop

• Firefox 58.0.1: *https://www.mozilla.org/en-US/firefox/all/

• trash the currently installed Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean reinstall
• install the new version that you have downloaded
• https://support.mozilla.org/en-US/kb/Installing+Firefox+on+Mac

Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data When you uninstall and reinstall or update Firefox, but make sure NOT to remove personal data When you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.