Excellent Addon. A quick suggestion: Rated 5 out of 5 stars

Excellent work, this works brilliantly! A note you may want to add, whether to your preferences, docs, or whatever else, is that stylish needs to have the about:config entry extensions.stylish.editor set to 1, not 0. This tells stylish to use a plain text box for the popup editor, and allows It's All Text to actually update.

This review is for a previous version of the add-on (