Rated 4 out of 5 stars
With FF 43 & Stylish 1.4, editor lost codemirror abilities and becomes just a plain text editor. But idea & features are great.This review is for a previous version of the add-on (0.8.4). This user has a previous review of this add-on.
this isn't tested/supported on Stylish v1.4, only v2.0.6 and up
Edit: you need to use SEM for code mirror editor (I may add support for it one day).