Just to be clear, Firefox doesn't have a validator built-in. It's just syntax highlighting, and it has recently been enhanced to use the same HTML5 parser as the rest of the browser. So yeah you can see some of the errors you might make in your markup but it's very far from what an actual HTML5 validator should do. It doesn't even report all parse errors (and isn't meant to).

I'm pointing this out because of a recent blog post by Mozilla dev Henri Sivonen which says: "Please do not advertise the new View Source implementation by saying that Firefox now has a validator in the View Source window."

(see full post here for all the details, there's a section named Limitations: http://hsivonen.iki.fi/view-source/)

Thanks for the clarification. I've mostly been using it to find some quick, obvious errors, such as tag mismatches.