Rated 5 out of 5 stars
This is one of the most helpful extensions in my day to day work, along with Firebug. It's one of the things that brought me back to Firefox since it's available exclusively for it and it does such a good job.
For a long time I could not use it (on Linux 64bit) mostly because of Firefox breaking extensions on every release. I'm very glad it works again.
One feature I wished it had is an offline validator for HTML5, but maybe this will come too once the standard is closer to completion.
All in all, a great extension. Thank you Marc for taking the time to create and maintain this :)