I can't find the Help Button for the test ???
I hope it can help you.
I appreciate a lot your work and attention.
Firebug teach to me a lot of thing to me
;-)
I can see the problem now. It's somehow related to how Stylish is binding in user styles. It uses data URLs.
We still need to break that down to a more simple test case, i.e. having an HTML file, that adds stylesheets using data URLs. This would make it easier to reproduce (without Stylish, FirePath or any other add-on). Though I don't know exactly yet how to do that. If somebody has an idea, please let me know.