Looks okayish, except for local files. You can set locals files, but for whatever reasons they don't respect their CSS. Works on the first tab, not on any other. Might work if the CSS would be inline, but who would want that? Guess it has something to do with local vs. addon-startup path or whatever...
I don't see a reason why it should work on the first tab and not in others. Please test with a fresh Firefox profile and report an issue on GitHub if the problem still exists.
