Should he have been fired?

what is relevant is weather the person posted this in their free time or that his post on takimag was as a writer for the site.

if it is the later then he might be projecting an image or message that does not follow the websites vision/image that it has made for itself. if that is the case then firing him is acceptable.
however if he posted this completely unrelated to his profession then there is no reason for him to be fired.

This is not a hypothetical abtract musing with insufficient information available to make a judgment. You have access to the site. Look for yourself at the contents of it and give an actual answer, not a text book if-then-else-maybe.