Download Firefox Browser App to MacBook Pro

There is a good page of instructions and various versions of Firefox

available depending on the vintage or configuration of your Mac:

• How to Download and Install Firefox on Mac - Firefox Help

https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Certain older versions have moved out of their final status ESR; as

such were no longer supported. For those affected systems. For

example El Capitan 10.11.6, options may include Waterfox browser

or Brave. ~ As time goes on, older macOS systems see no update.

The above linked page is merely a portal; from there, you may

choose to download something your Mac system could use.

Hopefully there's an update version Firefox. If not, see if the macOS

can be upgraded to newer version. If your Mac runs El Capitan, it

also may run High Sierra. Mine was an easy upgrade. Have backups

of your Mac system, from Time Machine on external USB HDDs.

Good luck & happy trails! ☃️🌿