Could you explain what kind of comments you're referring to? Is this with a Firefox feature or on a particular website, for example, and if it's a website, which one?

I most often reply to comments on Huffington Post, and it is on this site that I noticed the problem. They have a section called "Conversations", and right below the title there is a total of "Comments". Clicking "??? Comments" opens a box listing possibilities for ordering comments, "Social Ranking," "Chronological", or "Reverse Chronological" - newest first.

Unfortunately, that section of the page is pulled in from Facebook, and it seems Facebook is making no effort to remember your preference, even from article to article. I don't think there's any Firefox setting that can help with this, but maybe someone has created an add-on for it or documented some other workaround?

Note: I was not logged into Facebook while testing, so I can't say for sure that it wouldn't remember your preference if you were logged in.