Is the HTML shown via 'View Source' different from the HTML shown in (Firebug) developer tools?

Yes they are different.

View source shows you the original HTML source of the page.

The inspector shows you the DOM as it was interpreted by the browser. This includes for example changes made by javascript which cannot be seen in the HTML source.

Oh and this also counts for the developer tools that allow you to view the DOM in other browsers, like the Chrome Web Inspector and Internet Explorer Developer Toolbar. The HTML source will always be the same across all browsers, the generated DOM might differ as it is an interpretation and render engines are not all the same (unfortunately).

It should also be noted that aside from the dynamic DOM manipulation which could be occurring via javascript etc, Firefox will also parse and "clean-up" malformed (X)HTML, so these changes will also affect what you see when inspecting an element.