I'm using Firefox alongside Firebug Developer tools.
Is the HTML shown in View Source (Ctrl + U) different from the HTML I see when inspecting elements using Firebug?
What are the differences between the two?
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.
The difference are (but not limited to) that in firebug console you can see dynamically changes, which occurred in web page. For example, DOM elements modifications (from AJAX or regular JavaScript), CSS in-place modifications, etc...
In Firefox, View Source may reload the page, changing that is on it. See this bug and vote for it if it is bothering you:
来源：https://stackoverflow.com/questions/13070190/is-the-html-shown-via-view-source-different-from-the-html-shown-in-firebug-d