Support for this add-on will end with Firefox 56.
There is no possibility that this add-on can be updated to work as a WebExtension, which is the only extension type permitted to work with the release version of Firefox 57+. This add-on, like many which we all use on a regular basis, can not be implemented under the limited capabilities permitted to WebExtensions, which have a very limited ability to affect the Firefox user interface. If you wish to continue to use such add-ons, you will need to use Firefox 52 ESR, which will be supported by Mozilla through 2018-06-26.
The only other possibilities are to use Firefox Nightly, or perhaps Firefox Developer Edition, which can have support for other extension types enabled. However, Mozilla plans changes which will break many add-on interfaces in future Firefox releases. Thus, it's not clear for how long existing add-ons will function in Nightly and Developer Edition.
In addition, Mozilla has stated that they will, at some (unspecified) point, stop accepting updates for extension types other than WebExtensions.
Personally, for me, this change in Firefox means that Firefox is no longer Firefox. I call it Smolderfox, because the fire is basically out. Firefox without legacy add-ons is not the Firefox I've used for years. I may choose to continue to use Smolderfox, but it's already changing to a completely different browser. Thus, any inertia in my changing is broken. Many of the things I like about Firefox will be gone. The only reason which I had for continuing to use Firefox was that Mozilla used to have a human review every add-on from a security and privacy point of view. That's was a big reason. Unfortunately, Mozilla has chosen to switch to automatic programmatic review of add-ons. They have chosen to not have add-ons which were reviewed only by automatic checks to be distinguished in any way from those which have had a complete review (programmatic and human). Thus, add-on security is now equivalent to Chrome, which is abysmal.
If you use a significant number of folders to organize your bookmarks, then you have probably been frustrated with only having a few folders remembered by Firefox and displayed in the most recently used list. I know that for me it was routine for me to have a folder just drop of that list when I wanted to put another bookmark in that folder. Then instead of having the convenience of just selecting it from a short list of recently used folders, I had to navigate, again, to the folder I wanted. With this extension you can set the number of folders displayed in the recent folder list to the length of your choice. Upon installing this add-on the list will begin growing to the length you have specified (default 10). Folders to which you have navigated in the past were not stored beyond the old default of 5, so the list will not jump to being larger, but will grow as you access additional folders.
If you are using bookmarks quite a bit, you might also find Add Bookmark Here ² to be useful. I use both that extension and this one together.
As the developer of Change Bookmark Recent Folder List Length, I would like make sure it works for you. I routinely use this add-on with the current Firefox version and the current Firefox Developer Edition (i.e. the alpha version) and have tested on versions down to 26.0. Obviously, it is impossible for me to test it with the huge number of possible combinations of add-ons. If you find an issue with using this add-on, please post in the support thread on Mozilla Discourse so I can help you (the link is also available as the "Support site" to the right of this "About this Add-on" section).