And now for an related, annoying quirk. If you view source on a page, you get the source before any Greasemonkey changes have taken place. Which is often the exact opposite of what you want. I’ve worked around this with the “View Generated Source” function in the Web Developer Extension.
Bonus extension: Html Validator Firefox Extension. Displays a little icon on the bottom right of the screen which shows when a page has warnings or errors. It can show the total warnings and errors for the page but you need to enable this option. Double click the icon for more details.