Rated 3 out of 5 stars

I use(d) QuickFolders from the very beginning when it offered a very narrow set of functions, but between them, there was the most important two - easy selection of folder to be displayed and drag-and-drop of messages to the folders which had assigned tabs. All other additional functions except those which supplemented these two were of very limited value for me. In fact, they were rather something that cluttered the interface. Nevertheless, I accepted them - it's easy to ignore something that you don't have to use.

Unfortunately, as of some days ago, the situation changed diametrally, because the Author decided to make money using well known and rather not a user-friendly method (which could be named extortion by more blunt users).
Now - without any warning - if you update the extension you will get the version for which you have to pay a yearly fee, or use a free, extremely limited variant which rightfully could be named "demo version" as it allows to serve 10 folders only.
That's true that the fee for the limited paid variant which allows serving 25 folders is really symbolic. However, the fee for the unlimited version isn't symbolic. Especially in the light of the Author's declaration that the money is for preserving further support and expansion of the extension.

I consider the Author's decision unfair. In my opinion, almost all works on the extension expanding its functionality are not necessary for most users. Thus efforts put into support of such a basic variant would be negligible in comparison with the current version. A fair solution would be to split the extension to two: basic - frozen at the level covering functions related to displaing, creating and coloring folder labels, switching folders, and moving messages to them. The rest - including toolbar, filters, and many other functions and options (which I can't even name as I never use it) could be put into PRO, paid (subscription) version.

This review is for a previous version of the add-on (5.7.2). 

As I have stated repeatedly, the monetization is also necessary to keep my Add-on compatible with current versions of Thunderbird - you can easily install the previous version 5.6 and disable Add-on updates to use it for "free as in beer" forever.

Just note that I cannot keep fixing regressions caused by Mozilla for free - and the Pro Licenses do not make up for the cost. I had 3500$ of external development last year in order to make QF compatible with Tb78; there is ongoing work for being compatible with Tb91.

Most feature requests come from my users (and not just "PRO" users) and are heavily curated in order to benefit the maximum number of users. You cannot expect a complex Add-ons like this to be maintained for free as supporting and updating them + discussing with the Tb core team asking not to disable features and convert them to API functions (for example, mail extensions are still not allowed creating their own toolbar, so I am using "experimental" back end code) several days of work every week.

You can just pretend I walked away from that work and use the older version, no problem. That's not extortion. Most of my users expect free support, timely updates and that the Add-on doesn't stop working and that's what only monetization delivers.