Nice idea but... Rated 2 out of 5 stars
The extensions doesn't work in my case.
I set 90 days but then in History Library the folders like "Older than six months" are still here and full of pages.
Please Contact Me
The cleaning is done when your Firefox is idle for 15 seconds. In order to activate this start firefox and then wait 15 seconds (don't touch the mouse, don't type anything, literally stare at it doing nothing).
If this doesn't help, please contact me directly and I can help you get it working ( firstname.lastname@example.org )
Please contact me so we can work on fixing this.