Hi, I am very sorry to hear that! I want to make sure that the add-on works for everyone. You can try to reinstall the the add-on. You also need to make sure the right options are enabled (they could have reset during a Firefox update) or try to disable and enable the options again. If those did not work, please contact me with the following information: What exactly doesn't work anymore, the option to keep music videos as videos or the option to turn mixes to normal video? Did it stop working for every video or only for a few? In order for me to be able to help you (and anyone else who has that problem) please contact me.

You can do so either by opening an issue on the GitHub:
https://github.com/Joooohannes/Less-Mixes-for-YouTube

or via E-Mail:
johannes.ji1702@gmail.com

I really hope you reply so I can solve your issue!