My Adblock Plus dialogue boxes don't look like your examples. Nonetheless, after reading your support-mozilla link, I tried unchecking the "Fanboy's Social Blocking List" under "Filter preferences", which appears to have done the trick.

Thanks for your response, Daniel. Please see the next comment...

Adblock Plus doesn't seem to have this feature. However, if I turn off Adblock Plus, the "Follow" button re-appears. Unfortunately, I'm not prepared to have advertisements shoved in my face everywhere i go, nor am I prepared to have my browsing slowed to a crawl by the time it takes for ads to load, so it looks like I'll have to put up with limited functionality on Seeking Alpha and not add to the authors that I'm following. Thanks for pointing me towards the answer to this question.

Just did a re-boot and the up-date to Firefox 53.0 was successful, but still no "Follow" button.

The "Follow" button is absent on any article or author's page that I look at. Since you have mentioned Firefox, I'm using version 52.0.02. Firefox has attempted to up-date itself several times recently, but the up-date has failed each time.

I'll make it three. I can't "unfollow" an author, because my "follow/unfollow" button has disappeared too.