On a Google Chromebook, the easiest option is to install Firefox for Android using the Google Play Store. See Install Firefox on an Android device using Google Play for more information.

The alternative, if you are a little more tech savvy, is to install the Linux version of Firefox. However, that's a little more difficult than just using the Android version. See Install Firefox on Linux for more information.

Hope this helps.

Note that the screenshot shows that you were 'tricked' to install a Firefox version for Windows that possibly (likely) contains malware (Program: firefox.exe; Publisher: Flash Player). Since you are on Mac, this still wouldn't make sense.

See also:

• https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update