This One's a Keeper Rated 4 out of 5 stars

It sorts, in a well thought out, persistent manner. Simple to to the point.The interface for setting the sort-order of bookmarks is a little ham-fisted because it offers only Icons (really tiny, hard to decipher icons), so if you don't know 'em, you can't knowledgeably set 'em.

On the other hand, this simple, fairly straightforward add-on features an unmentioned toolbar button, as well as Tools menu entry, which allow you to choose the moment you want to sort. Choose the Autosort option and it's all set-it-and-forget-it functioning.

Kudos on recognizing that many of us like to organize (classify) bookmarks with separators within folders and for preserving our preference.

With respect to the Developer's reply below, if a tooltip had shown up while my mouse was on any of the icons, I wouldn't have mentioned (by inference) the lack of labels. After I read the reply, I tried again. No joy, but I appreciate the thought behind including the tooltip labels. Their absence on my screen is probably a stand-alone occurrence. (If the user doesn't know what's supposed to happen, there is an assumption that the feature doesn't exist.)

Over all, this utility is well thought out. Congratulations.

This review is for a previous version of the add-on (1.6). 

To know what the icons mean, there is a tooltip for every icon.
So, you just have to put your mouse over an icon a few seconds to see what it means.