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Norton (Mac) - Message: "Norton Extension could not be installed because it is not compatible with Firefox 57 or Nightly Firefox 58 version"
Older versions of Norton extension for Firefox do not work after you upgrade it to Firefox 57 on Mac OS 10.9.x. To fix this problem, manually remove the older version of Norton extension, and install the latest version.
For Norton Internet Security 5.10.x and Norton Security 6.x, only Norton legacy extensions are available that are not compatible with Firefox 57 or later.
Start Firefox.
On the top-right corner, click the Open menu icon, and click
.On the Legacy Extensions tab, next to Norton Security, click .
Start Firefox.
Visit the Norton Identity Safe page to automatically download and install the Norton extension.
To make sure that the add-on is installed, click the Open menu icon, and click
.On the Extensions tab, make sure that the Norton extension is installed and enabled.