If this bug report should be posted elsewhere, please let me know.
Yeah man post it here: https://discourse.mozilla-community.org/t/support-change-bookmark-recent-folder-list-length/2505
I think this guy here is having hte same problem. Post in his topic too, users lots of times work togather to make magic! https://discourse.mozilla-community.org/t/so-confused-how-do-i-fix-bookmark-list-length-extention/3834/4
I am the developer of Change Bookmark Recent Folder List Length. I did not see your post until @noitidart cross posted t to the official support thread.
Short answer: There is a new version, 1.2.1, in the review queue which is compatible with Firefox versions 26 - 42.*. You can install it from the list of all versions on the Mozilla add-ons site. As usual, all users will be automatically updated when version 1.2.1 is reviewed by Mozilla.
There is a bit more detail in the official support thread. I would appreciate if we could move any additional discussion about Change Bookmark Recent Folder List Length to that thread.
How to contact add-on developers/submit bug reports:
Each add-on has the ability to have small links on the Mozilla add-ons page to the right of the “About this Add-on” section to any or all of a support site, email the developer, or an add-on homepage. These links only show up if the developer has set them up. If they are available, these links are probably the best way to contact the developer (e.g. I get email anytime a post is made to the support site/thread). I choose to have a support thread for each of my add-ons as I feel that having a public discussion can be helpful to multiple users. If private information needs to be communicated, it is possible to use private mail from there.
Another possibility is that the add-on developer may have supplied a website link in their profile on the Mozilla add-ons site which may give a way to contact them.
If those methods fail, then posting as you have done here, is a good option. In addition, you could try posting on mozillaZine.
If those fail, you can write a review for the add-on on the Mozilla Add-ons site. The developer may be emailed whenever a review is posted to their add-on. However, as a developer, I find it a bit frustrating when a review is used to report bugs instead of the support site for multiple reasons. One reason is that reviews do not provide any way to get additional information from the person reporting the problem when additional information may be needed to duplicate the issue so it can be fixed.